基于AlGaN/GaN HEMT太赫兹探测器的340 GHz无线通信接收前端

利用天线耦合AlGaN/GaN HEMT太赫兹探测器的自混频和外差混频效应,分别设计并测试了340 GHz频段直接检波式和外差混频式接收机前端。通过接收机信噪比的测量和接收功率的定标,得到了两种接收机的等效噪声功率。直接检波模式下探测器的响应度约为20 mA/W,直接检波模式和外差混频模式下接收机的等效噪声功率分别约为?64.6 dBm/Hz1/2和?114.79 dBm/Hz。在相同的载波功率和接收信号带宽条件下,当本振太赫兹波功率大于?7 dBm时,外差混频接收的信噪比优于直接检波的信噪比。当本振功率大于0 dBm时,外差混频接收机表现出优良的解调特性,其信噪比高出直接检波接收机的信噪比10 dB以上。     

Design of wide-band optical heterodyne balanced mixer receivers

The optical heterodyne balanced mixer, or dual-detector receiver, offers significant advantages over a single detector receiver. Balanced mixer receivers are particularly attractive for use in optical heterodyne communication systems because they conserve local oscillator power and cancel excess intensity noise present in the local oscillator. Simple circuit models that illustrate the noise performance, small signal gain, and bandwidth of a balanced mixer receiver are developed. A figure-of-merit for receiver noise performance is also derived. An example design of a gigahertz bandwidth optical heterodyne balanced mixer receiver and the techniques used to characterize near-quantum-limited receiver performance are discussed.     

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.     

Influence of (semiconductor) laser linewidth on the error-rate floor in dual-filter optical FSK receivers

We discuss an error-rate floor in dual-filter optical FSK receivers in terms of the IF linewidth and the frequency shift between 0s and 1s, using a comprehensive model for the receiver which takes into account nonzero laser linewidth, noise correlation and signal overlap between the filters. We present calculated results for total error probability in terms of signal power, IF linewidth and frequency shift, illustrating the effect of the floor on receiver sensitivity, and we contrast it with a related effect in heterodyne ASK systems.     

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     

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.     

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; ifeta = 0.7, then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency andlambda = 1.5 mum, the above quoted numberers give 0.9 pW/ kHz foreta rightarrow inftyand 1.35 pW/kHz fornu = 0.7. The loop bandwidth required is also evaluated and found to be155 Deltanu, whereDeltanuis the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be2.26 cdot 10^{-3} R_{b}where Rbis the system bit rate. ForR_{b}=100Mbit/s, this leads toDeltanu < 226kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerateDeltanuup to 0.09 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth (Deltanu < 3 cdot 10^{4} R_{b}).     

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     

Error probability evaluation of optical systems disturbed by phasenoise and additive noise

A direct and efficient method for evaluation of the error probability of optical heterodyne receivers in the presence of phase noise is presented. A closed form expression for the statistics of the decision variable, including photodetector shot noise and thermal noise from electronic circuitry, is derived. The analysis assumes simple integrating filters in the receiver and is based on a power series expansion of the filtered phase noise. The error probability is calculated using a saddle point approximation which is numerically simple and gives accurate results. The optimal prefilter bandwidth for best phase noise rejection is easily determined     

ASK multiport optical homodyne receivers

Several types of ASK multiport homodyne receivers are investigated, and the impact of the phase noise and of the shot noise on these receivers is analyzed. The simplest structure is the conventional multiport receiver with a matched filter in each branch. This structure can tolerateDeltavT[deltavis the laser finewidth andTis the bit duration) of several percent with a small power penalty (3.6 percent for 1-dB penalty and 5.2 percent for 2-dB penalty). Optimization of branch filters of conventional multiport receivers does not help when the linewidth (and the penalty) is small but does improve the receiver performance for larger linewidths. The most important point of the paper is the novel wide-band filter-rectifier-narrow-band filter (WIRNA) structure, proposed and investigated here for the first time for optical communication systems. It is shown that the optimized WIRNA homodyne receivers are extremely robust with respect to the phase noise: the WIRNA tolerable value ofDeltavTis 3.6 percent for 1-dB penalty and more than 50 percent for 2-dB penalty. Thus, the WIRNA structure opens, for the first time, the possibility of constructing homodyne receivers operating at several hundred megabits per second with conventional DFB lasers without complicated external cavities. Under no-phase-noise conditions, all the multiport receivers investigated here have the same performance, which is identical to that of heterodyne ASK receivers. In addition, the optimized WIRNA receivers can tolerate tapproximately) the same laser linewidth as the heterodyne ASK receivers. Thus, the main difference between the WIRNA multiport homodyne and heterodyne receivers is that the former shifts the processing to a lower frequency range, in return for a more complicated implementation. This difference makes the WIRNA multiport homodyne receivers particularly attractive at high (say, several gigabit per second) bit rates.     

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     

Transformer-tuned front-ends for heterodyne optical receivers

We present measured noise spectra for heterodyne optical receivers with front-end tuning based on a T-equivalent circuit of transformer coupling. A noise spectral density below 6pA/?(Hz) over a 1-3 GHz bandwidth is demon-strated, even with parasitic capacitances totalling 2 pF.     

Homodyne detection of infrared radiation from a moving diffuse target

Experiments have been performed in which the radiation from a CO2laser was coherently detected after being scattered from a moving diffuse reflector. This is generally the configuration of an infrared laser radar. The power-spectral-density of the heterodyne signal was measured and its width was shown to agree with the calculated value based on a theoretical model for the process. Expressions are obtained for the ratio of heterodyne signal bandwidth to heterodyne frequency for the cases of focused radiation, unfocused radiation, and for a typical radar configuration. In most cases, the heterodyne signal is found to possess a narrow-band character. The probability density of the signal envelope was also measured for a known scatterer (providing Gaussian scattered radiation) and was found to be Rayleigh distributed, as expected. The power-spectral-density and envelope probability distribution provide information about a scattering medium or target which cannot be obtained from average-value measurements of the heterodyne signal-to-noise ratio. This information is useful for communications applications, infrared radar, and heterodyne spectroscopy experiments. Finally, a simple and direct method of obtaining information about the statistics of an incident radiation field (which does not involve photocounting) is discussed.     

Receiver analysis for synchronous coherent optical fiber transmission systems

A receiver sensitivity expression applicable for both PSK homodyne and heterodyne optical fiber transmission systems is derived taking account of polarization misalignment, reduced modulation depth, preamplifier thermal noise, power coupling ratio of the fiber coupler, local oscillator excess intensity noise, and reference phase errors. From a comparison of recent studies on system performance degradation due to laser phase noise a generalized expression relating beat linewidth to phase error variance for pilot carrier and Costas phase-locked-loop receivers is defined.     

Far infrared heterodyne detectors

The characteristics of far infrared detectors are reviewed. Three detectors, the InSb hot electron bolometer, the GaAs Schottky diode and the Josephson point contact junction, have been incorporated as mixers into sensitive heterodyne systems. The performances of existing heterodyne receivers/radiometers are described and compared. Other applications of submillimeter heterodyne techniques are discussed.     

Phase diversity techniques for coherent optical receivers

In the present state of the art, coherent optical receivers most often operate in the heterodyne mode. Here a photodiode-amplifier combination having bandwidth greater than twice the bit rate (B) is needed: indeed bandwidths considerably greater than2Bare preferably employed to ease design of the bandpass filter needed for noise limitation, and to avoid demodulator penalties in some modulation schemes. For the high bit rate systems now coming into service (560 Mbit/s-2.4 Gbit/s), the optical receiver design requirements become more stringent for coherent heterodyne operation. The various modes of "zero IF" operation, however, require only baseband receiver module bandwidth. The options available are either homodyne (phase locked) operation, or phase diversity (multiport) techniques. In this paper, we compare these options, and show that phase diversity techniques are capable of good performance for high bit rate coherent receivers. In phase diversity operation, not only is phase locking avoided, but also the necessary frequency locking does not have high stability requirements. Furthermore, there are advantages in operating with a small frequency offset from zero (of the order of 1 percent of the bit rate). An experimental receiver has been operated at 320 and 680 Mbit/s, demodulating both amplitude shift keying (ASK) and differential phase shift keying (DPSK). Operation with FSK is also possible. Sensitivities so far achieved of -47.5 dBm (320-Mbit/s ASK) and -42 dBm (680- Mbit/s ASK) with limited local oscillator power are capable of substantial improvement when higher power local oscillators and lower noise receive modules become available. Demodulation of DPSK at 320 Mbit/s has also been achieved and shows a measured receiver sensitivity improvement of over 4 dB over ASK at the same bit rate and local oscillator power. These practical results show clearly that phase diversity is a very realistic option for high bit rate systems.     

Decision-driven phase-locked loop for optical homodyne receivers: Performance analysis and laser linewidth requirements

Optical homodyne receivers based on decision-driven phase-locked loops are investigated. The performance of these receivers is affected by two phase noises due to the laser transmitter and laser local oscillator, and by two shot noises due to the two detectors employed in the receiver. The impact of these noises is minimized if the loop bandwidthBis chosen optimally. The value of Boptand the corresponding optimum loop performance are evaluated in this paper. It is shown that second-order phase-locked loops require at least 0.8 pW of signal power per every kilohertz of laser linewidth (this number refers to the system with the detector responsivity 1 A/W, dumping factor 0.7, and rms phase error 10°). This signal power is used for phase locking, and is, therefore, lost from the data receiver. Further, the maximum permissible laser linewidthDelta vis evaluated and for second order loops with the dumping factor 0.7 found to be 3.1 × 10^-4. Rb, where Rb(bit/s) is the system bit rate. ForR_{b} = 100Mbit/s, this leads toDelta v = 31kHz. For comparison, heterodyne receivers with noncoherent postdetection processing only requireDelta v = 0.72-9MHz forR_{b} = 100Mbit/s. Thus, the homodyne systems impose much more stringent requirements on the laser linewidth than the heterodyne systems. However, homodyne systems have several important advantages over heterodyne systems, and the progress of laser technology may make homodyning increasingly attractive. Even today, homodyne reception is feasible with experimental external cavity lasers, which have been demonstrated to haveDelta vas low as 10 kHz.     

Decision-driven phase-locked loop for optical homodyne receivers: Performance analysis and laser linewidth requirements

Optical homodyne receivers based on decision-driven phase-locked loops are investigated. The performance of these receivers is affected by two phase noises due to the laser transmitter and laser local oscillator, and by two shot noises due to the two detectors employed in the receiver. The impact of these noises is minimized if the loop bandwidthBis chosen optimally. The value of Boptand the corresponding optimum loop performance are evaluated in this paper. It is shown that second-order phase-locked loops require at least 0.8 pW of signal power per every kilohertz of laser linewidth (this number refers to the system with the detector responsivity 1 A/W, dumping factor 0.7, and rms phase error 10°). This signal power is used for phase locking, and is, therefore, lost from the data receiver. Further, the maximum permissible laser linewidthDeltanuis evaluated and for second order loops with the dumping factor 0.7 found to be3.1 times 10^{-4} cdot R_{b}, where Rb(bit/s) is the system bit rate. ForR_{b} = 100Mbit/s, this leads toDeltanu = 31kHz. For comparison, heterodyne receivers with noncoherent postdetection processing only requireDeltanu = 0.72-9MHz forR_{b} = 100Mbit/s. Thus, the homodyne systems impose much more stringent requirements on the laser linewidth than the heterodyne systems. However, homodyne systems have several important advantages over heterodyne systems, and the progress of laser technology may make homodyning increasingly attractive. Even today, homodyne reception is feasible with experimental external cavity lasers, which have been demonstrated to haveDeltanuas low as 10 kHz.     

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,     

Homodyne optical phase locking of resonant cavity coupledsemiconductor lasers

Two confocal Fabry-Perot cavity coupled semiconductor laser diodes (CFP-LDs) have been constructed for optical phase-locking experiments. Their FM noise suppression characteristics were calculated and compared with measurements of FM noise using an optical resonator as the optical frequency discriminator (FM noise suppression ratio 39 dB). Spectral linewidth was measured and evaluated, and frequency drift of the heterodyne signal in the time domain (20 kHz/s), was also measured. A simple linearized model of the optical feedback system was used for the calculations. Using two CFP-LDs, homodyne optical phase-locking experiments were performed. The performance of the optical phase-locked loop (OPLL) was evaluated by measuring and calculating the phase error variance. The calculation took into account the actual power spectral density of FM noise of the lasers employed in the OPLL. The phase error variance, considering infinite bandwidth, is 2.26×10^-2 rad². Total phase-locked power concentration ratio of the slave laser in the OPLL was 97.7%