Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving

In this paper, we investigate the impact of interferometer delay time in a 5 Gb/s optical double sideband-suppressed carrier (ODSB-SC) RoF system transmitting two wavelength interleaved radio frequency (RF) signals at 10 and 15 GHz over an optical fiber. Here, an optical Mach–Zehnder modulator is used for both optical carrier suppression and signal modulation. At the receiver, delay interferometer is used for the separation of RF frequency signals. We analyze the performance of the RoF system by varying the value of delay time of interferometer from 0.02 to 0.14 ns. The result shows that the RoF system performance is optimum for the time delay of 0.1 ns. Further, the optical spectrums, RF spectrums and eye diagrams of two interleaved RF signals have been compared.     

一种提取光反馈混沌激光器时延信息的谱分析方法

鉴于时延信息严重威胁着混沌通信的保密性,提出了一种利用频谱仪直接确定激光器时延信息的新方法。通过记录和观察信号的频谱,发现其频谱受到了时延信息的调制;进一步对频谱作逆傅里叶变换,即可直接获取光反馈混沌激光器的时延信息。实验中对腔长L=11.02 m的单反馈和腔长近似相等(L1=11.02 m, L2=11.25 m)或腔长成整数比(L1=11.02 m, L2=22.03 m) 的双反馈情形进行了研究。分析结果表明:在两外腔长度近似相等的情况下,2个外腔长度信息不能被隐藏;当反馈外腔长度成整数比时可以从频谱中提取出半导体激光器的反馈延迟时间。     

The Spatial and Temporal Evolution of the Glow in an RF Discharge

The temporal and spatial evolution of the glow in a 1.0- and 2.6-MHz radio-frequency (RF) excited discharge has been photographed with a high-speed framing camera. Evidence is presented showing electrons with a ballistic behavior in the body of the glow and a time delay between the maximum optical intensity of the glow and the maximum RF voltage. The effect of the dc self-bias on the glow is also shown. The implications of these observations on the dynamics of the ion motion in the plasma are discussed.     

Simple and highly accurate technique for time delay measurement in optical fibers by free-running laser configuration

A novel and simple technique for the measurement of time delay in the optical fiber by a free-running laser is proposed and demonstrated. The fiber to be measured was spliced to an erbium-doped fiber so as to form a ring-cavity laser. The mode beating frequency of the laser was measured to determine the round-trip time delay. This precise and efficient technique has an accuracy of 10(-8) for a fiber length of 100 km and of 10(-6) for a several-meters-long one.     

Flame propagation monitoring in an engine by an optical technique

A Schlieren laser technique has been used to investigate the transit time of the flame front at selected locations inside a running automotive engine. Thus we get information on the flame speed and the delay between ignition and formation of the front in a very simple fashion without disturbing the combustion itself. The only obvious requirement is that the engine has optical access, and the proposed technique can thus be used whenever other optical diagnostic techniques are used. We also present some examples of the results we obtained.     

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.     

Method to identify time delay of chaotic semiconductor laser with optical feedback

We propose a power spectrum analysis method to directly identify the time delay of a chaotic semiconductorlaser with optical feedback. By measuring the radio frequency (RF) power spectrum of the chaotic laserand performing an inverse Fourier transform, we can easily unveil all the time delay signatures, regardless ofwhether the chaotic output is induced by single or multiple feedback. This method successfully retrieves thetwo time delay signatures from the power spectrum of a semiconductor laser with double-cavity feedback.     

Intensity-modulated fiber Bragg grating sensor system based on radio-frequency signal measurement

An intensity-modulated, fiber Bragg grating (FBG) sensor system based on radio-frequency (RF) signal measurement is presented. The RF signal is generated at a photodetector by two modulated optical signals reflected from the sensing FBG and a reference FBG. Wavelength shift of the sensing FBG changes intensity of the RF signal through changing the delay between the two optical signals, with temperature effect being compensated automatically by the reference FBG. It also exhibits important features including potentially high-speed measurement, low cost, and adjustable sensitivity. In the experiment, strain measurement with a maximum sensitivity of -0.34 microV/micro epsilon has been achieved.     

High-efficiency hybrid brillouin/ytterbium fiber laser

A high-efficiency hybrid Brillouin/ytterbium fiber laser (BYFL) is demonstrated using a 41.5-cm-longhighly ytterbium-doped fiber and a 10-m-long single-mode optical fiber. The BYFL operates at 1 052.92nm, and the difference between it and the Brillouin pump (BP) wavelength matches the expected stimulated Brillouin scattering (SBS) Stokes shift. Its output power reaches 70.1 mW, which is more than seven times higher than the seeded BP power. The BYFL has an optical signal-to-noise ratio that is greater than 65 dB and has many potential applications, such as in controllable optical delay lines, sensing, and RF photonics.     

Fault-tolerant polarization-insensitive photonic delay line architectures using two-dimensional digital micromirror devices

A binary multichannel photonic delay line (PDL) module is introduced that gives balanced loss switched states and a polarization-insensitive operation via the use of binary operation Digital Micromirror Devices (DMDs). Experimental demonstration of a DMD-based PDL architecture is performed for a 6.84-ns time delay design. Experimental results include a 25-beam feed interchannel crosstalk test indicating a <−60 dB optical interchannel crosstalk level for a 0.381 mm interchannel distance in the multichannel PDL. An average optical signal-to-leakage noise ratio of 35.33 dB is measured for this PDL. A butterfly design PDL optical architecture is proposed for minimizing loss and improving assembly accuracy. These DMD-based variable PDLs can be used in applications ranging from radio frequency (RF) fiber-optic signal processing systems to adaptive optics for astronomical and laser radar arrays.     

Tunable photonic delay lines in optical fibers

A comprehensive overview is presented about optical fiber‐based tunable photonic delay lines, which have been steadily developed over the last decade for the realization of all‐optically controlled timing functions. The most widely used techniques, such as those based on slow & fast light and wavelength conversion associated to dispersion, are described and their physical limitations are discussed in terms of the maximal achievable delay, the associated signal distortion and signal bandwidth. Besides, an entirely different approach for all‐optical signal delaying is introduced. This technique is based on movable grating reflectors dynamically generated in highly birefringent optical fibers. This type of delay line has experimentally demonstrated large tunable delaying with a moderate signal distortion for high capacity optical data streams and even for wideband analog signals.     

利用先验估计自适应加窗的准静态超声弹性成像位移估计算法

准静态弹性成像技术是基于组织压缩前和压缩后超声回波射频信号进行组织运动重构的弹性成像技术。提出了一种基于先验估计的自适应窗函数算法,在位移估计过程中,使用已估计的临近窗的时延值作为先验信息,自动调整截取压缩后射频信号段的截取窗函数,提高了互相关运算所需的压缩前和压缩后信号段之间的相关性。仿真实验结果表明,该算法不仅大大提高了成像速度,而且提高了信噪比较低时的成像质量,同时该算法具有更宽的应变通带。      

表面粗糙度光纤传感器研究

基于光在粗糙表面的散射原理,利用时间延迟技术,设计了一种快速、非接触测量表面粗糙度的光纤传感器。与同类传感器相比,成本低,系统误差小。     

High-Accuracy Measurement of the Dispersion of Chirped Fiber Gratings

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Novel model of FM-noise conversion in a UMZI using RF external phase modulation of lasers: Theoretical and experimental results

In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement.     

Instantaneous frequency measurement using two parallel I/Q modulators based on optical power monitoring

A scheme for instantaneous frequency measurement(IFM)using two parallel I/Q modulators based on optical power monitoring is proposed.The amplitude comparison function(ACF)can be constructed to establish the relationship between the frequency of radio frequency(RF)signal and the power ratio of two optical signals output by two I/Q modulators.The frequency of RF signal can be derived by measuring the optical power of the optical signals output by two I/Q modulators.The measurement range and measurement error can be adjusted by controlling the delay amount of the electrical delay line.The feasibility of the scheme is verified,and the corresponding measurement range and measurement error of the system under different delay amounts of the electrical delay line are given.Compared with previous IFM schemes,the structure of this scheme is simple.Polarization devices,a photodetector and an electrical power meter are not used,which reduces the impact of the environmental disturbance on the system and the cost of the system.In simulation,the measurement range can reach 0 GHz-24.5 GHz by adjusting the delay amount of the electrical delay lineτ=20 ps.The measurement error of the scheme is better at low frequency,and the measurement error of low frequency 0 GHz-9.6 GHz can reach-0.1 GHz to+0.05 GHz.     

Picosecond flat-top pulse generation by low-bandwidth electro-optic sinusoidal phase modulation

We report the first experimental demonstration to our knowledge of a microwave frequency upshifting system based on phase modulation. A sequence of flat-top optical and RF pulses at a repetition rate of 18.22 GHz, each with a FWHM time width of approximately 25 ps, is generated from a sinusoidal RF tone of only 3.680 GHz, in good agreement with our analytical and numerical calculations. A simple explanation of this technique based on Talbot effect theory is provided. The practical limitations and capabilities of the phase-modulation-based frequency upshifting approach for ultrabroadband RF waveform generation are also discussed.     

Characterization of a supercontinuum generated from a photonic crystal fiber and its application to coherent Raman spectroscopy

The temporal and spectral profiles of supercontinuum radiation generated from a photonic crystal fiber are evaluated with a polarization-gate frequency-resolved optical gating technique. The supercontinuum is then applied to coherent inverse Raman spectroscopy. A stimulated Raman signal of cyclohexane is observed as an induced absorption signal with an instantaneous response. The Raman signal has a peak at a slight negative delay time, which is explained by perturbed Raman-induced coherence.     

Dynamic path length changes in all-fiber mirrors: Transmission modulation

In this paper, we present a technique to modulate the transmission of an all-fiber mirror. This technique is based on the phase modulation of the light in the fiber loop, combined with the time delay between the clockwise and anticlockwise propagating beams. Using Jones calculus, a theoretical analysis has been carried out to describe the effects of static polarization changes and a dynamic phase modulation. An experimental all-fiber optical mirror has been constructed, and using a 1-MHz piezoelectric disc as the phase modulator, we demonstrate that it is possible to achieve either a 1-MHz or 2-MHz transmission modulation by adjusting the polarization state.     

Improved analysis of dispersion compensation using differential time delay for high-speed long-span optical link

A dispersion-compensation technique using differential time delay has been analyzed for a high bit rate dispersive limited system using higher order dispersion terms. The technique is based on splitting the power spectrum into upper and lower parts, corresponding to the two modulation sidebands, and delaying one of these with respect to the other. RMS, phase deviation, dimension free chirp parameter, and figure of merit have been evaluated due to higher order dispersion terms for ideal and realistic optical communication systems. It has been shown that the transmission distance could be enhanced to fourfold, sixfold, and eightfold when the compensation has been performed using second-, third-, and fourth-order dispersion (20D, 30D, and 40D) terms.