An Approach to Photonic Generation of High-Frequency Phase-Coded RF Pulses

A novel method to generate high-frequency phase-coded RF pulses using all-fiber components is proposed. The system consists of a mode-locked fiber laser (MLFL), a dispersive element, an unbalanced Mach-Zehnder interferometer (UMZI), an optical phase modulator (PM), and a photodetector (PD). The PM is incorporated in one arm of the UMZI. In the system, an ultrashort pulse generated by the MLFL is broadened and chirped after passing through the dispersive element, which is then sent to the UMZI, to get two time-delayed chirped pulses. By beating the time-delayed chirped pulses at the PD, an RF pulse with its frequency dependent on the time delay difference is obtained. The generated RF pulse can be phase coded if an encoding signal is applied to the PM. A theoretical model is presented which is verified by experiments. The generation of RF pulses with binary phase coding is also experimented     

Analysis of a true time delay photonic beamformer for transmission of a linear frequency-modulated waveform

A generalized conversion matrix (GCM) and numerical analysis are used to study the distortions suffered by a linear frequency-modulated radio frequency (RF) pulse while propagating through photonic links to be used in wideband phased arrays. The analysis shows the effects of dispersion of all orders, coherent crosstalk and nonlinearity of the optical components on the RF pulse, and the high performance needed to achieve acceptable RF performance of the temporal (impulse) response. The effects of the electrical-to-optical (E/O) and optical-to-electrical (O/E) conversions are also considered. Using the GCM, the optical amplitude and phase fluctuations are converted into their RF counterparts, thereby reducing the optical problem into the well-understood RF domain. A photonic wavelength-controlled true delay device is experimentally shown to achieve good RF performance over a 4-GHz bandwidth, with predicted sidelobe levels below 30 dB.     

Photonic generation of BFSK RF signals based on optical pulse shaping

A novel method to generate binary frequency shift-keying(BFSK) radio frequency(RF) signals in optical domain is proposed.In the proposed system,an optical short pulse train is converted into super-Gaussian RF pulses with high frequency based on optical pulse shaping by two Mach-Zehnder fiber interferometers(MZIs).And the generated RF signals are coded using a fast electro-optic switch.By properly designing the MZIs,BFSK RF signals with desired code pattern and modulation index can be generated.A theoretical model for describing the system is developed,and the generation of BFSK RF signals in millimeter-wave regime is demonstrated via simulations.     

Coupled optoelectronic oscillators for generating both RF signaland optical pulses

We present experimental results of coupled optoelectronic oscillators (COEO) constructed with a semiconductor optical amplifier-based ring laser and a semiconductor colliding pulse mode-locked laser. Each COEO can simultaneously generate short optical pulses and spectrally pure radio frequency (RF) signals. With these devices, we obtained optical pulses as short as 6.2 ps and RF signals as high, in frequency, as 18.2 GHz with a spectral purity comparable with an HP83731B synthesizer. These experiments demonstrate that COEO's are promising compact sources for generating low jitter optical pulses and low phase noise RF/millimeter wave signals     

Generation of 40 GHz phase stable optical short pulses using intensity modulator and two cascaded phase modulators

Pulse sources based on lithium niobate modulators are very attractive for optical time division multiplexing (OTDM) transmission systems because the modulators are now commercially available,qualified for system use,and can operate up to very high speeds and over a wide wavelength range.In this paper,we describe the principles of operation and performance of the pulse source based on lithium niobate modulators.The pulse source is based on a Mach-Zehnder intensity modulator (IM) and two phase modulators (PMs).The continuouswave (CW) light is modulated in an IM and then strongly phase modulated in two cascaded PMs.The chirped pulses are subsequently compressed to desired width using dispersion compensation technology.This method has the advantage of acquiring larger chirp using normal PM rather than that special designed PM of very low Vπ.It can also generate shorter pulses than conventional methods incorporating only one PM driving by a radio frequency (RF) signal with the power larger than 1 W which may damage the device.Generation of 40 GHz optical pulses shorter than 2 ps is theoretically illustrated,simulated and experimentally verified.Experimental results show that 40 GHz phase stable optical pulses with pulse-width of 1.88 ps,extinction ratio (ER) larger than 20 dB,the timing jitter of 57 fs and signal-to-noise ratio (SNR) of 32.8 dB can be achieved.This is also a cavity-less pulse source whose timing jitter is determined only by the RF source rather than by the actively controlled cavity.In the experiment,the phase noise of the RF source we used is as low as -98.13 dBc/Hz at a 10 kHz offset frequency which resulting very low timing jitter of generated pulses.The pulses are then modulated at 40 Gbaud/s with an inphase/quadrature (l/Q) modulator and multiplexed to 160 Gbaud/s with less interference between each other.After back-to-back demultiplexing by an electro-absorption modulator (EAM) to 40 Gbaud/s and demodulation by a delay interferometer (DI),clear and opened eye diagrams of 40 Gbaud/s I and Q tributary signals are obtained which verify the good performance of generated pulses in the 160Gbaud/s differential quadrature phase shift keying (DQPSK) OTDM system and further prove the phase stability and high quality of generated pulses.     

Multiwavelength picosecond optical pulse generation using anactively mode-locked multichannel grating cavity laser

A multichannel grating cavity (MGC) InGaAsP-InP ridge diode laser is found to be suitable for multiwavelength ultrashort optical pulse generation using active mode-locking techniques. By using different configurations of the MGC laser, dual wavelength picosecond pulses are successfully produced with good spectral quality, either simultaneously or with a programmable relative delay between the channels. For a channel separation of 2.2 nm, pulses with a duration of 60 ps and a spectral width of 11.6 GHz have been obtained. The experimental results are compared with those from a theoretical analysis of the multiwavelength mode-locking process using a set of modified coupled-cavity rate equations. The minimum achievable pulsewidths generated by the actively mode-locked MGC laser are shown to be limited by the resolution bandwidth of grating used. The dependence of pulsewidth on RF drive frequency detuning is examined and it is found that the ultrashort pulses can be obtained over a wide range of frequency detuning. The interchannel cross-talk originating from gain saturation and carrier depletion is also discussed     

The Michelson resonator free-electron laser. I. Passive modestructure and mode decay

A general analysis of the evolution of phase locking in an RF linac-driven Michelson resonator free-electron laser (FEL) is presented. By providing a delay of one RF period in the secondary arm of the interferometer, successive optical pulses can be coupled at the beamsplitter, so that they build up from noise with a definite phase relationship. Phase locking is described in terms of longitudinal mode decay by extracting the mode losses directly from the passive frequency response of the resonator. The analyses predict significant mode structure simplification in microsecond macropulses, so that high-resolution spectroscopy is feasible on RF linac-driven FELs. Simulations of the perfectly tuned Michelson resonator FEL are described from spontaneous radiation to saturation, using a one-dimensional pulse propagation code. Excellent agreement with the analytical results in the small-signal regime is demonstrated     

Noise characterization of a regeneratively mode-locked fiber ringlaser

A regeneratively mode-locked fiber ring laser (RML-FRL) and an active harmonically mode-locked fiber ring laser (ML-FRL) have been characterized for both amplitude and phase noise by investigating the detected RF spectra of the optical pulse trains. Quantification of noise in the optical pulses reveals that the stability of the RML-FRL in terms of noise performance is superior to its ML-FRL counterpart. The optical pulse noise was measured over a frequency band of 100 Hz to 100 kHz and it was found that the pulse amplitude noise reduced from 0.6% in the ML-FRL to 0.3% in the RML-PRL. The total rms noise in the detected optical pulses from the RML-FRL improved by more than 30% compared to that measured for the ML-FRL, with a phase noise improvement of 15 dB at 100 kHz offset frequency from the carrier. An rms timing jitter of 0.38 ps was estimated in the optical pulse train from the ML-FRL, which reduced to 0.26 ps in the RML-FRL. In addition, complete elimination of the relaxation oscillations noise spikes in the detected RF spectrum of the optical pulses from the RML-FRL has been observed     

基于光移相器的全光正交频分复用技术研究

为了克服光正交频分复用系统中傅里叶变换和数模转换实现的＂电子瓶颈＂,达到提高系统传输速率的目的,采用光延迟器和光移相器构建全光离散傅里叶逆变换（IDFT）和离散傅里叶变换（DFT）的方法,设计一个3路全光IDFT/DFT模块,利用Optisystem软件平台仿真实现一个3×40Gb/s的全光正交频分复用实验系统,并分析系统性能,获得发射光脉冲宽度与系统传输误码率的关系,得到了光脉宽越大误码率越大的结论。     

Self-phase modulation and spectral broadening of optical pulses insemiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems     

Generation of super-stable 40 GHz pulses from Fabry-Perot resonator integrated with electro-optic phase modulator

A simple technique for converting a continuous-wave laser beam into a stable pulse train with high repetition frequency is demonstrated. The generated train is based on the synthesis of its Fourier spectrum which is composed of higher-order sidebands produced by a Fabry-Perot resonator integrated with an electro-optic phase modulator. Super-stable 40 GHz, transform-limited optical pulses are successfully generated.     

RE Pulse Shapes for Selective Excitation in NMR Imaging

It is shown that the requirements of an RF pulse needed to produce a slice for use in NMR imaging approximate the requirements of the windows used for calculating the discrete Fourier transform (DFT). This approach gives an interesting insight into the physical background of pulse shape tailoring. One of the more successful DFT windows is the 3 term Blackman-Harris window, which is tailored to produce minimum sidelobes. The results of applying radio frequency (RF) pulses modulated to give this window shape is given. The pulse shape produces a uniform magnetization over a range of frequencies with a quasi-linear phase shift making it suitable for producing a slice for NMR imaging when an echo is produced via an inverse magnetic field gradient.     

Analytical Characterization of Optical Pulse Propagation in Polarization-Sensitive Semiconductor Optical Amplifiers

In this paper, we perform analytical characterization of optical pulses propagating through a polarization-sensitive semiconductor optical amplifier (SOA). We derive analytical expressions for the carrier density, gain and phase evolution along the SOA and show how these expressions prove useful in optical signal processing applications. The propagation of counter-propagating pulses as well as pulse streams across SOAs have been analysed and expressions for energy gain have been derived in all these cases. We also show that our analytical results reduce to corresponding results of polarization insensitive SOAs already published. The analytical results are in excellent agreement with detailed numerical simulations done in MATLAB using the NIMROD portal. The analytical calculations lead to significant savings with regard to simulation time and processing capacity requirements. We further prove that the energy gain difference for counter-propagating pulse streams is directly proportional to the difference delay between in them and hence can be used as a measure of the delay difference. This theoretical result agrees well with experimental results.     

基于LFM-OTDR的超长跨距光纤电力通信链路监测

为了降低电力通信系统中白噪声和随机多径噪声,提出了2种基于线性调频-光时域反射仪(LFM-OTDR)技术的监测方案,即单频LFM-OTDR方案和频分复用LFM-OTDR(FDM-LFM-OTDR)方案。该监测方案采用LFM光脉冲信号作为探测信号,利用分数阶傅里叶变换将后向散射回来的探测信号变换到分数阶傅里叶域进行信号处理和散射光功率计算,从而能更加有效地分离探测信号和后向噪声。实验结果表明:与单脉冲OTDR方案相比,采用单频LFM-OTDR方案、FDM-LFM-OTDR方案使OTDR的动态范围分别提升了约8.3 dB、11 dB。     

Group delay and α-parameter measurement of 1.3 μmsemiconductor traveling-wave optical amplifier using the interferometricmethod

The group delay dispersion and the linewidth broadening factor (α-parameter) are measured for a 1.3-μm semiconductor traveling-wave optical amplifier (TWA) using a newly developed interferometric method. By Fourier transforming an interferometric cross-correlation signal, both the optical phase and the gain are simultaneously obtained in the entire gain bandwidth. The group delay spectrum is evaluated from the frequency derivative of the phase, and by selecting an appropriate interval for the interferometer scan, a refractive index dispersion of ~ 3.2 fs/nm is separated from the dispersion caused by Fabry-Perot resonance. From the phase and gain change with injection current, the α-parameter spectrum is evaluated, and the results indicate a strong dependence on wavelength     

Research of photonic-assisted triangular-shaped pulses generation based on quadrupling RF modulation

We report the fabrication and optical properties of sub-micrometer-thick Ge₂₀Sb₁₅Se₆₅chalcogenide rib waveguides.The radio-frequency（RF）magnetron sputtering method is used to deposit 0.83μm-thick films.A protective layer of SU-8 is employed to prevent the attack of the alkaline developer,and CHF₃ is used as the etching plasma for reactive ion etching（RIE）.Finally,the resulted rib waveguides with smooth sidewalls and vertical pattern profiles are rendered.The propagation losses for 4μm-wide waveguides are measured to be 0.7 dB/cm for transverse electric（TE）modes and 0.68 dB/cm for transverse magnetic（TM）modes at 1 550 nm via the cutback method.     

Chirped RF Pulse Generation Based on Optical Spectral Shaping and Wavelength-to-Time Mapping Using a Nonlinearly Chirped Fiber Bragg Grating

Chirped radio-frequency (RF) pulse generation based on optical spectral shaping and nonlinear wavelength-to-time mapping in a nonlinearly chirped fiber Bragg grating (NLCFBG) is investigated. In the proposed approach, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is shaped by an optical filter that has a sinusoidal frequency response. The spectrum-shaped optical pulse is sent to the NLCFBG, to implement nonlinear wavelength-to-time mapping. A chirped electrical pulse with the central frequency and chirp rate determined respectively by the first- and second-order dispersions of the NLCFBG is then obtained at the output of a high-speed photodetector. An approximate model that describes the chirped RF pulse generation is derived, which is verified by numerical simulations. Chirped pulse generation with a pulse compression ratio as high as 450 is demonstrated. The key device in the chirped RF pulse generation system is the NLCFBG, which is investigated in detail with an emphasis on the influence of its group delay ripples on the performance of the pulse generation system. Techniques to design and fabricate the NLCFBG are also discussed. The proposed approach provides a potential solution for the generation of chirped RF pulse with a high central frequency and large chirp rate for applications in pulse compression radar systems.     

Electronically controlled high-speed wavelength-tunable femtosecondsoliton pulse generation using acoustooptic modulator

The compact system of electronically controlled high-speed wavelength-tunable femtosecond (fs) soliton pulse generation is realized for the first time using a passively mode-locked fs fiber laser, a polarization maintaining optical fiber, and an acoustooptic (A-O) modulator. The wavelength of the output pulses can be continuously tuned simply by controlling the input voltage into the A-O modulator. The wavelength of the soliton pulses can be changed at 2.5-μs intervals. Wavelength stabilization, time division wavelength multiplexed soliton pulse generation, and a wavelength scanner have been demonstrated     

10 GHz超低抖动光短脉冲源

报道了一种新型的重复频率为10 GHz,脉宽为5.3 ps,抖动为184 fs的高稳定光短脉冲源.将大信号调制半导体激光器产生的10 GHz光脉冲,先送入LiNbO3电光相位调制器增强负啁啾,并使光谱进一步展宽,再通过色散补偿光纤(DCF)压缩脉冲啁啾,可得到光短脉冲.由于大信号调制激光器输出的光脉冲本身具有负啁啾,而通过相位调制器的光脉冲在不同的时间间隔内既有正啁啾也有负啁啾,通过适当调整进入相位调制器的光脉冲时延,使其通过相位调制器后累加产生更大的负啁啾,再利用正色散光纤压缩啁啾,从而得到低抖动且无基座的光短脉冲.     

Tunable Terahertz Signal Generation by Chirped Pulse Photomixing

Tunable signal generation in the gigahertz to terahertz range was demonstrated by photomixing of a chirped optical pulse and its time-delayed replica. The chirped pulses were created by chromatically dispersing femtosecond pulses from a mode-locked fiber ring laser, and frequency tuning was achieved by adjusting the time delay and/or the amount of group velocity dispersion. It is shown that the signal phase is locked to the femtosecond-laser pulses and that an extinction ratio close to 100% can be obtained. Analytical calculations on the signal chirp introduced by higher order dispersion are also presented and compared with the experimental data