Linewidth-insensitive coherent AM optical links: design,performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems     

The use of optically coherent detection techniques for true-timedelay phased array and systems

We analyze a true-time-delay, optically controlled phased array antenna system whereby beam forming is accomplished using a large number of antenna elements that can receive any of several different microwave true-time-delays via the use of coherent optical carriers transmitted through a single fiber. The transmitter and receiver architectures are described in detail. We present calculations for the signal-to-noise ratio (SNR), the signal-to-interchannel interference ratio (SIR) and dynamic range (DR) of one channel for various modulation-demodulation schemes (i.e., AM-heterodyne, FM, PM). We show for an example system that can accommodate 128 antenna elements and provide 128 different delays (7 bits), assuming AM modulation with a bandwidth of 1 MHz, a transmitter power of -5 dBm and a laser linewidth of 10 MHz, we can obtain DR≃50 dB for one channel using a channel separation twelve times of that of the microwave frequency (f_m). For the FM scheme, DR≃60 dB can be obtained for one channel with a laser linewidth of 1 MHz, a FM modulation index of one, and a channel spacing of about 13f_m. With a local oscillator laser tuning range of 100 Å, the system can provide seven-bit resolution for a 128 element antenna array. It is found that coherent optically controlled phased array antennas provide improved controllability over direct detection method, and can have the SNR, SIR and DR that meet the stringent requirements of modern high resolution microwave antenna systems     

采用NPRO光源的零差相干光通信实验

在空间相干光通信应用中,针对传统激光器线宽较宽、相位噪声大、易导致锁相环路失锁的问题,研制了单频Nd:YAG非平面环形（NPRO）激光器,其线宽小于1 kHz,相对强度噪声（RIN）低于-150 dB/Hz,具有窄线宽、低噪声的特点。搭建了光锁相环路,在信号光功率-67 dBm的情况下实现了两台NPRO激光器的相位锁定。在此基础上开展了信号频率为10 MHz和1.25 GHz的模拟通信实验,在信号光功率分别为-60 dBm和-53 dBm时可观测到较理想的眼图。在2.5 Gbps数字通信实验中,接收灵敏度达到-50 dBm,此时误码率为3.210-6。系统灵敏度可接近量子极限,明显优于传统的IM/DD方式,是一种适合长距离、大容量传输的空间通信方式。     

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,     

Optically coherent direct modulated FM analog link with phase noisecanceling circuit

This paper demonstrates a new phase noise canceling (PNC) circuit for use with an optically coherent analog frequency modulation (FM) link employing directly modulated distributed feedback (DFB) lasers. Direct frequency modulation of a semiconductor laser is a highly efficient optical-to-electrical conversion process, which can lead to very low noise figure (NF) and high dynamic range (DR). However, the large laser phase noise found in semiconductor lasers significantly degrades the FM link performance. The PNC circuit is a simple means for canceling the laser phase noise while extracting the modulated signal and taking advantage of the high FM conversion efficiency of semiconductor lasers. The theoretical performance of the PNC PM link is discussed in terms of the signal-to-noise ratio (SNR), NF, and DR, and is compared to a Mach-Zehnder modulated link which uses high-power, solid-state lasers. Phase noise cancellation is demonstrated in an experimental PNC FM link. Comparison of a PNC FM link to an externally modulated AM (coherent) link shows a 31 dB improvement in the NF of the FM link, and a 10 dB improvement in DR. However at higher received optical powers phase-to-intensity noise limits the performance of both links to well below the theoretical calculations     

窄线宽低噪声可调谐非平面环形激光器

针对空间相干光通信和探测等应用,对非平面环形激光器的线宽、噪声和调谐特性进行了系统的实验研究。单频输出功率达到752mW,光光效率42%,斜率效率54%。采用延时自外差拍频法测试了激光线宽,其随泵浦功率的增加而增大,输出功率小于200 mW 时,线宽小于1 kHz,在最高输出功率下线宽为2.3 kHz。激光强度噪声主要由弛豫振荡引起,相对强度噪声（RIN）随着泵浦功率的提高而降低。在1.78W 泵浦功率下,RIN 达到-93 dB/Hz。采用温度和压电两种方式进行了激光调谐。温度调谐范围达到62 GHz。压电调谐范围达到130MHz,响应带宽100 kHz。     

Coherent transmission of 60 FM-SCM video channels

A fiber-optic 60-channel FM video system using coherent subcarrier multiplexing (SCM) is described. Two receiver implementations were studied. For the 60-channel coherent experiment, a carrier-to-noise ratio of 17 dB and a corresponding signal-to-noise ratio of 56 db was achieved with a detected optical power of -32 dBm and a phase modulation index of β-0.07. This represents a 15.2-dB improvement over a similar 60-channel direct-detection FM-SCM system and a 5.6 dB improvement compared to the performance of the system when a semiconductor optical preamplifier was used     

Impact of local oscillator intensity noise on performance of coherent subcarrier multiplexing system using common local oscillator (CLO)

The impact of local oscillator intensity noise on the performance of a coherent subcarrier multiplexing (CSCM) system using a common local oscillator (CLO) is analysed. For a given LO power, RIN noise, and thermal noise, the CNR can be expressed in terms of x, the ratio of sigma /sub sh//sup 2/+ sigma /sub th//sup 2/ over sigma /sub RIN//sup 2/ and, for x<<1, the CNR decreases as LO power increases; for x>>1, the CNR increases as the LO power increases; as x approaches 1, there exist an optimum value of local oscillator power and a corresponding maximum CNR. For a system with RIN=-150 dB/Hz, P/sub S/=-20 dBm, beta =0.02, N=40, NF=3 dB and B=10 MHz, the (P/sub LO/)/sub opt/ is found to be -4 dBm and (CNR)/sub max/ 22.5 dB.<>     

Simultaneous measurement of the linewidth, linewidth enhancementfactor α, and FM and AM response of a semiconductor laser

Using a commercially available computer controlled spectrum analyser with tracking generator, optical input section, and optical delay line it is possible to measure the linewidth, linewidth enhancement factor α, and FM and AM response of a semiconductor laser in one process. The determination of the linewidth yields also information about the frequency noise density and the determination of a delivers information about the nonlinear gain. Assuming an optical input power of 0 dBm, a laser linewidth <50 MHz and a modulation response of the laser without cut-off, AM indices m>0.01% and FM deviations of about >10 MHz up to 20 GHz can be detected     

S/N and error rate evaluation for an optical FSK-heterodyne detection system using semiconductor lasers

Receiving performance is evaluated for an optical FSK-heterodyne detection system in which semiconductor lasers are used as both an FSK transmitter and an independent local oscillator. Noise and error rate are measured under feedback stabilization of IF signal frequency and electrical equalization of semiconductor laser FM modulation frequency characteristics. The minimum received signal power of -44 dBm, which is about 2 dB better than that in IM direct detection, is achieved at a 10^-9error rate for a 200 Mbit/s signal. Excess errors for FSK signals result from frequency broadening of the laser spectrum. Both AM and FM quantum noises in the lasers are primary factors which determine system performance. Error rate characteristics in an optical FSK direct detection system, in which a Michelson interferometer is employed as an optical frequency discriminator, are compared with the above results.     

Densely spaced FDM coherent star network with optical signalsconfined to equally spaced frequencies

The results obtained with a fiber-optical star network using densely spaced frequency-division-multiplexing (FDM) and heterodyne detection techniques are discussed. The system consists of three optical sources transmitting around 1.28 μm, frequency-shift keying (FSK) modulated at 45 Mb/s and spaced by 300 MHz. A 4×4 optical coupler combines the three optical signals. The FDM signals, received from one of the four outputs of the coupler, are demultiplexed by a heterodyne FM receiver. The minimum received optical power needed to obtain a bit error rate (BER) of 10^-9 is -61 dBm or 113 photons/bit, which is 4.5 dB from the shot noise limit. Cochannel interference is negligible for the above channel spacing and modulation rate. The results indicate that such a system has a potential throughput of 4500 Gb/s. The results obtained with two frequency stabilization circuits used to confine these three FDM optical signals to a comb of equally spaced frequencies are also presented     

High performance 1.5 mu m multiple-quantum-well DFB-DC-PBH-LDs at 30 mW output power for Gb/s range coherent systems

Long cavity (1200 mu m) MQW-DFB-DC-PBH-LDs have been developed. The devices simultaneously showed less than 1 MHz spectral linewidth, lower than 15 kHz FM dip frequency, moderate FM efficiency (250-300 MHz/mA at 100 MHz modulation frequency), as well as 10-GHz FM bandwidth (6 dB down level from the peak), at 30 mW output power condition. Such LDs will be suitable for Gb/s continuous phase frequency shift keying (CPFSK) current optical fiber communication systems.<>     

External optical feedback effects on intensity noise ofvertical-cavity surface-emitting lasers

The authors have experimentally evaluated the effect of external optical feedback on the relative intensity noise (RIN) of a vertical-cavity surface-emitting laser (VCSEL) diode. In the absence of optical feedback, the smallest RIN is found to be -135 dB/Hz. For optical feedback levels approaching -25 dB, the RIN is degraded by about 20 dB. The authors have measured feedback-induced power penalties in a 500-Mb/s intensity-modulation/direct-detection (IM/DD) link and determined that the penalty exceeds 1 dB if the optical feedback ratio is larger than -30 dB     

卫星相干光通信多普勒频移开环估计技术

卫星相干光通信在国外已经接近工程应用阶段,主流的传输接收方式为BPSK调制/零差探测,接收端用光锁相环（OPLL）进行载波恢复。但即使在今天,光锁相环的实现依然有难度,而且存在捕获时间过长等现象。文中借鉴了现在的研究热点技术光纤通信中的频偏估计、载波相位恢复技术,针对星间激光通信中存在GHz量级大多普勒频移的工程背景,提出了一种基于DSP的频偏开环补偿算法。从理论上分析了该算法的原理,重点研究了激光器噪声和接收机噪声对频偏估计精度带来的影响,并通过仿真进行了验证。本振光功率为10 dBm时,当接收功率高于-47 dBm,误码率优于10-4;将接收功率提高10 dB后,频偏估计标准差小于370 kHz。     

An IF-to-Baseband Σ∆ Modulator for AM/FM/IBOC Radio Receivers With a 118 dB Dynamic Range

A single-bit fifth-order complex continuous-time IF-to-baseband SigmaDelta modulator for AM/FM/IBOC receivers is presented. The input IF is 10.7 MHz and the sampling frequency is 41.7 MHz. The modulator achieves a dynamic range of 118dB in AM mode (3 kHz BW), 98dB in FM mode (200 kHz BW), and 86dB in IBOC mode (500 kHz BW). The modulator's high dynamic range enables the realization of an AM radio receiver without a VGA and without an AM channel-selection filter, thereby reducing system complexity and cost. The elimination of the VGA also improves the sensitivity and the overall noise figure of the receiver. The modulator's spurious free dynamic range is 88 dB in the bandwidth from 25 to 525 kHz. The IM2 distance is 92 dB, and the IM3 distance is 91 dB. The ADC was fabricated in a one-poly five-metal 0.18-mum CMOS process with an active area of 6.0mm². It consumes 210 mW from a 1.8-V supply     

Submillimeter wave generation through optical four-wave mixingusing injection-locked semiconductor lasers

This paper presents a comprehensive analysis of submillimeter wave generation through optical four-wave mixing (FWM) in an injection-locked semiconductor laser. The probe wave introduces pump-probe difference frequency amplitude modulation (AM) and phase modulation (PM) of the slave laser injection locked to the pump wave. The AM and PM indexes of the locked laser output lightwave modulated at the submillimeter wave frequency have been calculated. The submillimeter wave power generated through this technique has also been calculated theoretically. The analysis predicts a submillimeter wave power of -3.7 dBm at 300-GHz frequency for a free-running slave laser output power of 30 mW. In the presence of amplitude modulation of the probe wave, the same modulation is transferred to the submillimeter wave. The suppression of output lightwave amplitude noise relative to the pump, probe, or free-running slave laser amplitude noise has also been estimated in this analysis. At a submillimeter wave frequency of 300 GHz and a probe power equal to the pump power, typical amplitude noise reduction occurs by 7 dB     

非线性光谱展宽载波的湍流信道高速传输特性

采用非线性光谱展宽光源作为高重复频率部分相干光载波光源,开展大气湍流信道中高速信息传输实验研究.通过皮秒光纤激光器泵浦高非线性光纤获得非线性光谱展宽的部分相干光载波光源,重复频率可达10 GHz.采用10 Gbit/s脉冲高速数字调制,通过模拟大气湍流信道传输,相干光载波和部分相干光载波在湍流信道传输前后的信噪比分别从...     

AM and FM Noise of BARITT Oscillators

AM, FM, and baseband noise of a BARITT diode oscillator in the range 100 Hz-50 kHz off the carrier has been measured under various operating conditions. A simple calculation has been made, relating the baseband noise to the oscillator AM and FM noise via measured amplitude and frequency modulation sensitivities and the results have been compared with the noise measured. It is shown that, depending on the bias current applied, both AM and FM noise performance can be degraded by up-conversion. Complete removal of up-converted noise requires a high-impedance low-noise bias supply since both the diode noise and bias supply noise at baseband frequencies may be significant when up-converted. Even with all modulation suppressed, the AM and FM noise has a flicker component almost completely correlated with the diode flicker noise at baseband frequencies. The RF power dependence of the AM and FM noise has also been investigated. It is shown that the BARITT oscillator noise compares very favorably with that of IMPATT's and TEO's. Values of -142 dB/100 Hz (AM noise) and 3.5 Hz/(100 Hz)/sup 1/2/ for Q/sub ext/ = 200 (FM noise) have been measured at 30 kHz off the carrier.     

一种高性能CMOS单片中频接收机

研制了一种CM O S低压低功耗中频接收机芯片,它包含混频器、限幅放大器、解调器以及场强指示、消音控制等模块,可用于短距离的FM/FSK信号的接收和解调。该接收机采用1st s ilicon 0.25μm CM O S工艺,芯片的测试结果表明整机接收灵敏度为-103 dBm,最高输入射频频率可以达到100 MH z,解调器的线性解调范围为±10 kH z,典型鉴频灵敏度为40 mV/kH z,输入FM信号(调频指数3,信号频率1 kH z)时解调信号的SFDR为41.3 dB。芯片的工作电源电压范围为2~4 V,工作电流3 mA,有效面积0.25 mm2。     

Direct fibre-optic transmission of entire microwave satellite antenna signals

The entire 500 MHz bandwidth multicarrier C-band microwave signal spectrum from a satellite antenna is transmitted over low-loss optical fibre. The optical system introduces 1dB or less of degradation for both low-spectral-density QPSK signals and high-spectral-density FM video signals present in typical satellite transmission systems. For the video signals, the signal/noise ratio after AM conversion was reduced from 56 dB to 55dB.