Quantum-limited operation of balanced mixer homodyne and heterodyne receivers

The quantum-limited operation of balanced mixer optical homodyne and heterodyne receiver using a 50-50 percent beam splitter and two identical photodetectors is experimentally confirmed. The absolute quantum noise levels are calibrated with an incoherent GaAs light-emitting diode output. The photoelectron statistics, obtained through the analog photon counting experiment, are reduced to the quantum limit. The photocurrent fluctuation spectral density measured with a spectrum analyzer also agrees with the quantum noise spectral density. Furthermore, the deviations from the absolute quantum noise level are within ±0.02 dB. Finally, complete suppression of local oscillator excess noise is demonstrated in the 0-1.5 GHz frequency region.     

Monolithic integrated coherent receiver on InP substrate

The fabrication of a monolithic integrated coherent receiver with a wavelength-tunable DFB laser as local oscillator, a 3-dB waveguide directional coupler for mixing, and p-i-n photodiodes for detection is discussed. Optical heterodyne detection with a clear beat signal was experimentally observed using this monolithic integrated coherent receiver. Since an n-type substrate was used in this device, the two p-i-n photodiodes were not implemented in a balanced mixer configuration. Balanced mixing might be possible if the same structure were fabricated on a semi-insulating substrate. The results obtained suggest the possibility of applying this type of monolithic integrated coherent receiver to optical communication systems     

Performance implications of time delay and responsivity mismatchfor balanced CPFSK lightwave receivers

The performance implications of time delay mismatch and photodiode responsivity mismatch are assessed for balanced CPFSK heterodyne receivers with differential detection. The receiver sensitivity is determined using a technique which combines computer simulation for characterizing the signal at the receiver output with a formula-based method of evaluating the bit error ratio. This approach permits consideration of laser phase noise, local oscillator intensity noise, nonlinear signal processing, and nonideal components. The numerical results quantify the penalty in receiver sensitivity due to mismatch, for different levels of local oscillator intensity noise. It is determined that time delay mismatch primarily affects the intensity noise contribution to the IF signal, while responsivity mismatch primarily affects the received signal component of the IF signal     

Improved design of tuned optical receivers

We present new design principles for improved heterodyne tuned optical receivers where several tuning inductances reduce the influence of thermal receiver noise over a broad frequency range. A theoretical example for a 600 MHz tuning bandwidth shows a reduction of thermal receiver noise (and thus in required local oscillator power) of up to 13dB. The example is tested experimentally for the so-called mixed tuning configuration. We obtain good agreement with the theoretical predictions. The experimental RMS noise current is <5pA/?(Hz) over a 580 MHz bandwidth with the lowest value of 3-5 pA/?(Hz) at 950MHz.     

Intensity noise in ASK coherent lightwave receivers

The effect of local oscillator intensity noise on the performance of two and three-branch ASK homodyne receivers and single-branch ASK heterodyne receivers is investigated and an optimum local oscillator power is found. At optimum local oscillator power, both the three-branch and heterodyne receivers are found to have a somewhat better sensitivity than the two-branch receiver. If the local oscillator power is high than the optimum value, the three-branch receiver is significantly less sensitive to intensity noise than the other two receivers     

Sensitive 50-Ω coherent lightwave receiver utilizing ahigh-power local oscillator

The performance of a simple 50-Ω balanced receiver suitable for use in multigigabit heterodyne coherent lightwave systems is described. A high-power, tunable, distributed-Bragg-reflector laser is used as a local oscillator, resulting in a thermal noise penalty of only 2.4 dB. Frequency-shift keying (FSK) system results at 1 and 4 Gb/s are reported     

Investigation of a heterodyne receiver with open structure mixer at 324 GHz and 693 GHz

The performance of a submillimeter heterodyne receiver using an HCOOH laser local oscillator and an open structure mixer with a Schottky barrier diode has been optimized for 693 GHz. Working at room temperature a single sideband (SSB) system noise temperature of 7,300 K, a mixer noise temperature of 6,100 K and a conversion loss of 12 dB has been achieved. The same receiver system has been investigated at 324 GHz using an HCOOD laser local oscillator yielding a noise temperature of 3,100 K (SSB), a mixer noise temperature of 2,400 K (SSB) and a conversion loss of 10 dB (SSB). An acousto-optical spectrometer has also been constructed, with 1024 channels and a channel-bandwidth of 250 kHz. The system NEP per channel was 2.5×10^?17 W/Hz^1/2 at 324 GHz and 5.0×10^?17 W/Hz^1/2 at 693 GHz.     

A monolithic GaAs IC for heterodyne generation of RF signals

An integrated heterodyne signal-generating GaAs chip is reported. This circuit contains: an on-chip local oscillator with external inductors tunable by means of on-chip variable capacitors from 2.1 to 2.5 GHz; a doubly balanced mixer with associated drive circuitry; and an IF preamplifier. The circuit delivers +6 dBm (equivalent 50 Ω) into the designed load impedance of 200 Ω with -30-dBc harmonic distortion over a 1.4-GHz 3-dB bandwidth. Circuit elements presented include: a unique variable-threshold limiter, a self-biasing push-pull oscillator, doubly balanced mixer, and a self-biasing unity-gain phase splitting amplifier.     

A 345 GHZ heterodyne receiver for the James Clerk Maxwell Telescope

In this paper we describe the design and performance of a low-noise 345 GHz heterodyne receiver. The mixer uses a lead alloy SIS tunnel junction mounted in reduced height rectangular waveguide and is tuned with a single backshort. Local oscillator power is provided by a broad-band Gunn oscillator which drives a frequency quadrupler. The heterodyne performance has been verified in the laboratory using a gas absorption cell. In November 1991 this receiver was successfully commissioned and by direct comparison with a Schottky diode receiver we confirm a best receiver noise temperature of 150K (DSB) at 355 GHz and a tuning range of 300 to 380 GHz. The receiver is now available as a JCMT facility instrument.     

A dual-detector optical heterodyne receiver for local oscillator noise suppression

The performance of a dual-detector optical heterodyne receiver was analyzed and compared with the performance of a conventional single-detector heterodyne receiver. The dual-detector receiver is found to offer two main advantages over the single-detector receiver-1) increased performance in the presence of local oscillator intensity fluctuations that might severely degrade single-detector receiver performance, and 2) decreased local oscillator power requirements. These two advantages are particularly important in a communication system which uses semiconductor laser diodes as local oscillators. Such lasers suffer from intrinsic wide-band intensity fluctuations and can also impose strict power constraints on receiver design. Based on the analysis, suggestions for the optimal design of a dual-detector heterodyne receiver are made. Also, several experiments were performed to demonstrate the improved performance of the dual-detector receiver-both for unguided- and guided-wave receivers.     

Optical and electronic mixing in an avalanche photodiode

Application of a local oscillator to an avalanche photodiode enables it to be used as an electronic mixer when demodulating optical signals varying in amplitude. In an optical heterodyne receiver the diode performs both optical and electronic mixing. Experiments are described which show that very low conversion losses are possible. The method has significant advantages over more conventional techniques.     

Impact of phase noise in weakly coherent systems: a new andaccurate approach

Coherent optical systems for future broadband local loops may use lasers with significant phase noise, manifest as broad linewidths. This phase noise can be accommodated if the receiver is correctly designed, i.e. if nonsynchronous (envelope or square-law) IF demodulation is used and sufficient IF bandwidth is provided. It is difficult to analyze the performance of a coherent optical receiver when the signals are corrupted by phase noise. The central theoretical problem arising from filtering a signal with phase noise is defined in a particular form which permits the derivation of the forward or Fokker-Planck partial differential equation for probability density of the output voltage of the receiver. The results are used to discuss the IF bandwidth required for optical heterodyne receivers for amplitude-shift-keying (ASK) signals     

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 μW. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 μW, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB·Hz^2/3, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 μW, and a local oscillator power at the photodetector of 228 μW. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,     

30μm Heterodyne receiver

Advantages and constraints of remote measurements using heterodyne spectroscopy near 30 μm are discussed. The state of the art of wideband HgCdTe photomixers and PbSnSe diode laser local oscillators being developed for far infrared heterodyne receivers is described. The first compact 30 μm heterodyne radiometer was built and initial results at 28 μm show about 2% mixer efficiency for a 500 MHz bandwidth receiver. Factors limiting receiver performance are discussed, along with the projected sensitivity of new interdigitated-electrode HgCdTe photoconductor mixers being developed for operation up to 200 μm.     

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.     

Optical amplifier noise figure in a coherent optical transmissionsystem

Two important aspects of optical amplifier noise figure as measured with a heterodyne detection receiver are investigated. First, differential mode gain will result in polarization-dependent degradation of the perceived amplifier noise figure, even when the noise figures for the TE and TM modes of the amplifier are the same. For a differential mode gain of 7.5 dB the noise figure degradation can be as large as 3 dB, and experimental data is reported in good agreement with theoretical predictions. Secondly, the first experimental demonstration of the use of image-rejection techniques to improve the sensitivity of a heterodyne receiver limited by beat noise between the local oscillator and amplified spontaneous emission is discussed     

A Compact 60-GHz Transmitter--Receiver

A new, compact, low-cost, and reliable 60-GHz transmitier-receiver was developed for civilian use. Common components are used for transmitting and receiving functions. An IMPATT oscillator generates the millimeter-wave output power for both the transmitter and the receiver local oscillator. A common antenna is also used for transmitting and receiving signals without a circulator. A mixer is used as a modulator in transmission as well as a receiver front end. A noise figure of 13 dB is obtained by a balanced mixer with a 200-MHz IF frequency differential preamplifier. A reliable packaged GaAs varactor diode is used for the mixer-modulator (MM).     

A self-diplexing quasi-optical magic slot balanced mixer

Drawing on the principles of operation of the magic tee balanced mixer, a quasi-optical magic slot balanced mixer is proposed. The mixer exploits a diametrically fed annular slot radiator which radiates two orthogonal polarizations which are used as signal and local oscillator input ports. The idea has been tested at microwave frequencies. The measured radiation impedances, radiation patterns and conversion efficiency show that reasonable performance is readily obtained over a 30% bandwidth and that this quasi-optical balanced mixer is suitable for submillimeter wave applications     

5.2--Infrared 10.6-micron heterodyne detection with gigahertz IF capability

Analyses and experiments have been carried out on heterodyne detection at 10.6 microns with the objective of obtaining IF bandwidth capability into the microwave region. UHF and microwave measurements on the quantum-noise-limited generation-recombination (G-R) noise spectrum of compensated copper-doped germanium photoconductive mixer elements, measured under operational conditions at 10.6 microns, have shown response to beyond 2 GHz. The experiments were carried out directly at 10.6 microns using a mixer geometry and circuit arrangement intended to yield large mixer conversion gain and IF bandwidth. Engineering design equations are given for noise equivalent power (NEP) and mixer conversion gain (G) in terms of such parameters as IF noise factor, carrier transit time, carrier lifetime, mixer resistance, local oscillator power, dc bias power, etc. An expression for quantum-noise factor (QF) is defined. Graphs are also presented showing the effect on NEP,G, and QF of various parameters, and the tradeoffs possible to achieve high-frequency IF capability. An alternative approach is presented in which mixer conversion gain is calculated directly from the mixerI-Vcharacteristic in a manner analogous to microwave mixers.