涡旋光平衡探测系统转速测量精度与信噪比分析（特邀）

对于基于叠加态涡旋光和涡旋光零差探测等传统转速测量方式,光的远距离传输和发散等原因造成的信号光衰减会导致探测系统无法准确提取信号,而涡旋光平衡探测系统可以解决这一难题,但是以往的研究对该探测系统的精度和信噪比鲜有分析,这一定程度上限制了其工程化的进展。首先将零差探测系统作为对比项,通过分析不同转速下涡旋光平衡探测系统和零差探测系统测量精度的变化情况,证明了二者均可实现高精度测量,其次通过对比在不同信号光功率下二者的信噪比(SNR),发现了在测量微弱信号时涡旋光平衡探测系统具有明显优势;最后,通过分析不同本振光功率对信噪比造成的影响,揭示了平衡探测信噪比和本振光功率之间的关系,阐明了信噪比随本振光功率变化的原因。     

2μm双平衡式外差探测IQ解调与信噪比研究

建立了双平衡式外差探测IQ解调的数学模型,通过旋转λ/4波片来改变IQ信号间的相位差。从本振光强度过剩噪声、散粒噪声和热噪声三个方面对平衡式外差探测系统的信噪比(SNR)进行了仿真。仿真结果表明,当旋转λ/2波片使分束比在(0～0.272)和(0.728～1)范围内时,单源探测信噪比比平衡式探测的信噪比高;而分束比在(0.272～0.728)时平衡式探测的信噪比高于单源探测。搭建了2μm双平衡式外差探测实验系统,当分束比为0.5时,平衡式外差探测的信噪比比单源外差探测的信噪比提高10dB以上,从而证明了双平衡式外差探测系统在微弱信号检测中的可行性和优越性。     

Nonlinear Sagnac interferometer switch and its applications

Ultrafast all-optical switching based on the optical Kerr effect in a Sagnac interferometer which consists of a dichroic polarization-maintaining fiber coupler and dispersion-shifted polarization-maintaining fiber loop is reported. This nonlinear Sagnac interferometer switch has the advantage of high stability originating from completely balanced interfering arms. In addition, because dispersion-shifted fibers were used, increases in switching power and switching time were prevented. Moreover, polarization fluctuation was completely suppressed due to the all-polarization maintaining fiber configuration. The required switching power for complete switching was measured to be 1.8 W for a 200-m-long fiber. All-optical time division demultiplexing and logic operations, including inversion and operation, using the nonlinear Sagnac interferometer switch were successfully demonstrated at 5 Gb/s     

50-dB nonlinear crosstalk suppression in a WDM analog fiber system by complementary modulation and balanced detection

We investigate, theoretically and experimentally, a novel scheme to suppress the nonlinear crosstalk between wavelengths in wavelength-division multiplexing (WDM) analog fiber systems. It is based on an improvement to a previously introduced crosstalk-suppression technique by complementary modulation of two closely spaced optical twin carriers (TCs). By combining the modulated TCs by a balanced detector (TCs-balanced detection [TC-BD]), the suppression can be improved further. We have obtained up to 50-dB crosstalk reduction at low modulation frequencies, where crosstalk is most severe, and in the most interesting region for cable television systems.     

Linear and nonlinear chip-rate minimum mean-squared-error multiuserCDMA detection

A linear Kalman filter detector for code-division multiple access proposed earlier in the literature is extended to a structure that can handle arbitrary detection delays, through the mechanism of state augmentation. Because pre-detection RAKE combining is used in the detector, it is optimal for multipath channels, unlike the previous structure that performed post-detection combining. We also derive nonlinear Kalman detectors, which approximate the highly complex nonlinear minimum mean-squared-error detector, using the concept of “additional observations.” Both linear and nonlinear detectors require processing at one or more times the chip rate, and knowledge of the spreading codes of interfering users. They have the advantage over many other multiuser detection algorithms of not requiring the spreading codes to be periodic at the symbol rate, or matrix inversion. In addition, two of the detectors are able to generate and update a posteriori probabilities of the transmitted symbols, making them interesting for iterative multiuser detection     

Experimental Demonstration of Optical DQPSK Receiver Based on Frequency Discriminator Demodulator

This paper presents experimental data demonstrating a novel differential quadrature phase-shift keying receiver based on optical frequency discriminator demodulator with direct detection. The experimental results confirm theory, showing a factor of 2$times$ enhanced tolerance to chromatic dispersion compared with a conventional delay-interferometer-based demodulator with balanced detection. The frequency discriminator direct detection receiver also shows a significant improvement in tolerance to Gordon–Mollenauer nonlinear phase noise.      

Balanced distributed-element phase shifter

We present a balanced distributed-element phase shifter based on a nonlinear transmission line (NLTL) structure. Results show the second harmonic is more than 13dB lower at 0-V bias than a conventional single-ended NLTL phase shifter. We fabricated both balanced and conventional NLTL phase shifters with the same coplanar waveguide (CPW) design and diodes, and observed that phase shifting and insertion loss for both structures were quite similar, yet harmonic distortion was greatly improved in the balanced structure.     

Scattering of Plain Waves by the System of Balanced Vibrators with Nonlinear Loads under the Influence of Nonlinear Radar

Analysis of the secondary fields radiated by an ensemble of balanced vibrators with nonlinear loads under the influence of the radiation of nonlinear radar. The possibility of taking into account the empirically detected effect of the onset of negative differential resistance on the current-voltage characteristic of silicon diode structures under the influence of a relatively high level of the microwave power of nonlinear radar     

Start-Bit Detection and Gate-Pulse Generation Using Phase-Modulated Preamble for Asynchronous Packet Processing

Start-bit detection and a single gate pulse generation scheme scalable to high-speed operation for asynchronous packet processing have been achieved. It is based on phase-modulated two-bit preamble for a coder and one-bit-delayed detection combined with a balanced photodetector as exclusive or (xor) function for a decoder. By using pi/2 phase-shifted first-bit and a pi/2 phase-shifted one-bit delay line, a single gate pulse can be generated with high speed, regardless of packet period. We have successfully achieved the single gate-pulse generation with bit rate of 10.72 Gb/s and various packet periods from 12.8 to 120 ns. Amplitude of the gate pulse was almost constant with fluctuation of plusmn5%. Suppression ratio of a gate pulse could be improved up to 9.5 dB via nonlinear absorption characteristics of an electroabsorption modulator. Autonomous gating operation for an optical digital-to-analog converter was also verified for two-bit operation     

Noise characteristics of nonlinear semiconductor optical amplifiersin the Gaussian limit

This paper addresses noise properties of nonlinear semiconductor optical amplifiers. From a basic point of view, noise properties of nonlinear optical amplifiers are sufficiently different from those of linear amplifiers to warrant detailed modeling which has not been formulated previously. From a practical point of view, nonlinear semiconductor optical amplifiers are important for future all-optical signal-processing applications which may involve the operation of these devices in a saturated regime. Nonlinear amplifiers are also common in systems operating near 1300 nm and in integrated booster amplifiers. Under nonlinear operating conditions, amplifier noise contains a narrow-band contribution that comes about due to the nonlinear coupling of noise and gain. The more conventional broadband spontaneous noise also changes as the inversion factor becomes power-dependent and varies along the amplifier axis. We analyze noise in nonlinear amplifiers in the Gaussian limit (meaning, for fields consisting of large photon numbers) for CW or NRZ modulated signals and separately for short pulses. We consider the case of a single input as well as configurations of multi-input signals interacting via four-wave mixing. Using a specific detection system for the calculations of electronic signal-to-noise ratios, we demonstrate a reduction in the narrow-band electronic noise due to saturation in the single input case. We also demonstrate a vast advantage of using short pulses in four-wave-mixing applications     

Carrier detection of PSK signals

This paper reports on a theoretical study of the detection of the 2f and 4f carrier components of phase-shift keying (PSK) signals produced by passing signal and noise through a nonlinear device. Unbalanced quadrature PSK (QPSK) signals, i.e., QPSK signals with unequal power in the two channels, are studied. The complete range of channel power ratio is covered, with equal emphasis on the general unbalanced case and the two limiting cases of binary PSK and (balanced) QPSK. Analytic expressions are derived for the detected SNR of carrier harmonics 2f and 4f as a function of SNR, channel power ratio, and normalized input bandwidth. The results apply equally well to PSK data signals and direct-sequence spread-spectrum signals. Measurements confirm every aspect of the theory. The least detectable signal type is balanced QPSK, which is detectable (at 4f) at a threshold SNR ranging from -2 to -13 dB as the detection process gain (chip or data rate/detection bandwidth) is varied from 40 to 80 dB     

Accurate computations of the nonlinear phase noise and BER in DPSK modulation-balanced direct detection high-speed optical fiber communication systems

This article analyzes the nonlinear phase noise caused by an amplified spontaneous-emission of erbium-doped optical fiber amplifier and self-phase modulation (SPM) in a differential phase-shift keying modulation- balanced direct detection optical fiber communication system. Using numerical methods, the characteristic function of the differential nonlinear phase noise cos(Dj){\cos (\Delta \varphi)} in the received electrical current of signals and the accurate computation formula of the BER are achieved when SPM is dominant comparing to cross phase modulation and four wave mixing. The result is verified by a simulation of a 40-Gbit/s transmission system.     

Impact of filtering on RZ-DPSK reception

We discuss the influence of optical and electrical filtering on the performance of beat-noise limited balanced and single-ended direct detection of return-to-zero differential phase-shift keying (DPSK). Our simulations, supported by 40-Gb/s measurements, show that balanced DPSK detection outperforms both its single-ended equivalent and ON-OFF keying by /spl sim/2.7 dB, with higher gains at narrower optical filter bandwidths.     

Combating/eliminating dispersion-induced fading using suppressed carrier link for OFDM signal

We investigate the performance of 20-GHz radio over fibre (RoF) system having orthogonal frequency division multiplexed (OFDM) as radio signal using two modulation techniques – balanced detection for intensity modulation direct detection (IMDD) and coherent heterodyne detection suppressed carrier (SC) modulation. Dispersion-induced power fading is seen in conventional IMDD links due to the dependence of dispersion over frequency-dependent refractive index of the fibre. SC link is seen to compensate the power fading by terminating the direct current and even-order harmonics with the suppression of carrier along with balanced detection.     

偏振模色散对200 Gbit/s零路径色散控制光纤链系统的影响

基于一种二阶和三阶色散都作了完全补偿 (路径平均色散为零 )的光纤级联系统模型 ,用数值法研究了偏振模色散对皮秒光脉冲传输的影响。在零路径色散补偿系统中 ,线性色散得以完全补偿 ,为了减小非线性效应引起的脉冲窄化 ,系统必须保持较小的功率 ,但这样会影响系统的信噪比。计算结果表明 ,偏振模色散与非线性效应相互平衡 ,可使系统在较高的功率下保持脉冲宽度基本不变 ,从而获得较好的传输性能     

强度防护型激光防护机制

论述了非线性吸收、非线性折射、非线性散射等基于非线性效应以及基于相变效应的激光防护机制,分析了每种防护机制的优势和局限,指出了强度防护型激光防护技术的发展趋势。     

本振功率对目标微动激光平衡外差探测的影响研究

面向远距离目标微多普勒效应的探测,建立了本振光功率变化对激光平衡外差探测回波信噪比影响的数学模型,并进行了仿真分析。通过搭建的1 550 nm激光平衡外差探测实验平台测试了由于本振功率变化对于目标微多普勒特征提取效果的影响。研究结果表明,对于平衡外差探测,由于两光电探测器量子效率不可能完全匹配,考虑到散粒噪声、热噪声和本振相对强度噪声的影响,将存在一个最佳本振功率值使探测信噪比达到最大。当选取该本振功率进行探测时,获得的目标微多普勒特征可读性更强,提取误差更小。     

用于高速微弱光信号的平衡探测技术研究

长距离高速微弱光电探测技术在激光通信中应用广泛。为了在未被噪声湮没的环境中检测出所需的微弱信号,采用平衡探测技术对单管探测和平衡探测进行对比,并且对具体的平衡探测器电路进行了理论分析和仿真,得出了在一致性系数较大(k>0.6)时平衡探测器的信噪比优于单管探测的结论。从理论上分析了单管和双管探测的最小可探测光功率。结果表明,把平衡探测技术用于高速微弱光信号探测具有很大的优越性。     

Adaptive space-time feedforward/feedback detection for high datarate CDMA in frequency-selective fading

We investigate linear and nonlinear space-time minimum mean-square-error (MMSE) multiuser detectors for high data rate wireless code-division multiple-access (CDMA) networks. The centralized reverse-link detectors comprise a space-time feedforward filter and a multiuser feedback filter which processes the previously detected symbols of all in-sector users. The feedforward filter processes chip-rate samples from a bank of chip-matched filters which operate on the baseband outputs from an array of antennas. We present an adaptive multiuser recursive least squares (RLS) algorithm which determines the MMSE adjusted filter coefficients with less complexity than individual adaptation for each user. We calculate the outage probabilities and isolate the effects of antenna, diversity, and interference suppression gains for linear and nonlinear filtering and for CDMA systems with varying levels of system control (e.g., timing control, code assignment, cell layout). For eight users transmitting uncoded 2-Mb/s quadrature phase-shift keying with a spreading gain of eight chips per symbol over a fading channel with a multipath delay spread of 1.25 μs, the performance of a three-antenna feedforward/feedback detector was within 1 dB (in signal-to-noise ratio per antenna) of ideal detection in the absence of interference. By training for 10% of a 5-ms frame, RLS adaptation enabled the same detector to suffer less than a 0.5-dB penalty due to the combined effects of imperfect coefficients and error propagation. The advantage of nonlinear feedforward/feedback detection over linear feedforward detection was shown to be significantly larger for a CDMA system with enhanced system control     

Optical reflectometry with micrometer resolution for theinvestigation of integrated optical devices

An optical time-domain reflectometry (OTDR) system using an infrared subpicosecond pulse source in conjunction with balanced heterodyne detection is discussed. Experimental results show a density of more than 100 dB and a resolution of 60 μm in air. Taking advantage of the large tuning range of the laser system, it is possible to improve the resolution to less than 10 μm. The applicability of the OTDR system for the diagnostics of integrated optical devices is demonstrated for a simple GaAs waveguide structure