Optical processing based on spectral line-by-line pulse shaping on a phase-modulated CW laser

We demonstrate optical processing based on spectral line-by-line pulse shaping of a frequency comb generated by phase modulating a CW laser and show various applications including CW-to-pulse conversion, width/wavelength tunable return-to-zero pulse generation, pulse-to-CW conversion, wavelength conversion, and microwave photonics. The correlations between optical spectra, time-domain waveforms, and RF spectra are quantitatively studied.     

Quantitative Study of Optical Frequency Noise to Intensity Noise Conversion in Line-by-Line Pulse Shaping

We report the first quantitative study of intensity noise induced in line-by-line pulse shaping in response to time-varying changes in the comb frequency offset. Controllable comb linewidth broadening is synthesized through frequency dithering of a continuous-wave laser that is fed to a phase modulator. An electrical spectrum analyzer is used to examine the current power spectra of shaped time-domain intensity waveforms subject to comb frequency noise. A theoretical model predicting a 20 dB/decade scaling relation between the dither-induced noise and the frequency dither amplitude is presented. A numerical simulation method capable of predicting the precise form of the RF power spectrum in the presence of optical frequency dithering is explained. Two line-by-line shaping cases are considered in detail. Experimental data are in excellent agreement with the simulated results down to frequency dithers of a few tenths of a percent of the comb spacing. Tolerances to laser frequency fluctuations are given for several simple pulse shaping examples. The effect of pulse shaper parameters is also discussed.      

Synthesis of Millimeter-Wave Power Spectra Using Time-Multiplexed Optical Pulse Shaping

Millimeter-wave (MMW) electrical power spectra are flexibly synthesized by integrating fast wavelength switching, optical frequency comb generation, spectral line-by-line pulse shaping, and optical-to-electrical conversion. Control over generated RF power spectra is exercised through the choice both of the optical parent waveforms and of the wavelength switching patterns. Discrete or comb-like MMW power spectra are generated using periodic wavelength switching, while nearly continuous MMW spectra can be obtained when wavelengths are switched according to a pseudorandom bit stream.     

基于光频梳的超短光脉冲的产生及其在光模数转换中的应用

提出并实验验证了一种基于光频梳的超短光脉冲的产生方法。使用强度调制器和相位调制器级联直接调制直流激光得到了29条顶部功率变化小于1.5dB的光频梳。利用单模光纤色散将光频梳整形成重复频率为10GHz,脉宽为2.68ps的光脉冲。并成功用于对1～4GHz信号采样,系统的信噪比可达33.83dB,等效于5.33bit的有效比特数。     

Flat Frequency Comb Generation With an Integrated Dual-Parallel Modulator

A dual-parallel Mach-Zehnder modulator is employed to generate an optical frequency comb. Flat spectral response is obtained by optimizing the dc bias and the RF drive parameters of the device which is driven in the gigahertz frequency range. We demonstrate theoretically and experimentally, a flat spectral output across nine sidebands. A tunable multiwavelength source based on this approach allows for very precise wavelength spacing for dense wavelength-division-multiplexing applications and has the advantages of efficiency and simplicity.     

Ultra-Wideband Waveform Generator Based on Optical Pulse-Shaping and FBG Tuning

We propose and demonstrate experimentally a prototype for ultra-wideband (UWB) waveform generator based on optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs frequency-to-time conversion. A U.S. Federal Communications Commission (FCC)-compliant power efficient pulse shape is inscribed in the frequency domain by a fiber Bragg grating (FBG) with an excellent match between optimized and measured pulses. Two other popular UWB pulse shapes (Gaussian monocycle and doublet pulses) are achieved by proper tuning of two FBG-based variable optical filters. A balanced photodetector removes an unwanted rectangular pulse superimposed on the desired waveform, assuring compliance at low frequency.     

Photonic Polarity-Switchable Ultra-Wideband Pulse Generation Using a Tunable Sagnac Interferometer Comb Filter

A photonic approach to polarity-switchable ultra-wideband (UWB) pulse generation is proposed. The proposed setup principally consists of an electrooptic phase modulator (EOPM) and a tunable optical comb filter constructed with another EOPM inside a Sagnac interferometer. The comb filter working at its linear region performs the phase-to-intensity conversion to generate UWB pulses with monocycle shape. By adjusting the bias voltage of the EOPM inside the Sagnac interferometer, a shift of the comb filter can be achieved, which is the key point for a pulse polarity switch. A pair of monocycle pulses with reversed polarities was experimentally obtained by applying the dc bias of 0 and 1.9 V, respectively. The convenience to electrically control the pulse polarity is favorable for biphase modulation in UWB impulse radio applications.     

Chirped Microwave Pulse Generation Based on Optical Spectral Shaping and Wavelength-to-Time Mapping Using a Sagnac Loop Mirror Incorporating a Chirped Fiber Bragg Grating

In this paper, we propose and demonstrate an approach to optically generating chirped microwave pulses with tunable chirp profile based on optical spectral shaping using a Sagnac loop filter incorporating a chirped fiber Bragg grating (CFBG) and linear wavelength-to-time mapping in a dispersive element. In the proposed approach, the optical power spectrum of an ultrashort optical pulse is shaped by a CFBG-incorporated Sagnac loop mirror that has a reflection spectral response with a linearly increasing or decreasing free spectral range. The spectrum-shaped optical pulse is then sent to a dispersive element to perform the linear wavelength-to-time mapping. A chirped microwave pulse with the pulse shape identical to that of the shaped spectrum is obtained at the output of a high-speed photodector. The central frequency and the chirp profile of the generated chirped microwave pulse can be controlled by simply tuning the time delay in the Sagnac loop mirror. A simple mathematical model to describe the chirped microwave pulse generation is developed. Numerical simulations and a proof-of-principle experiment are implemented to verify the proposed approach.      

Programmable shaping of femtosecond optical pulses by use of128-element liquid crystal phase modulator

Programmable shaping of femtosecond pulses by using a 128-element liquid crystal modulator to manipulate the phases of optical frequency components which are spatially dispersed within a grating-and-lens pulse shaping apparatus is described. This apparatus makes possible gray-level control of the spectral phases and allows modification of the pulse shape on a millisecond time scale under electronic control. Refinements in the design of the multielement modulator result in pulse shaping fidelity comparable to that which can be achieved with microlithographically fabricated masks. Several examples of pulse shaping operation, including pulse position modulation, programmable pulse compression, and adjustable cubic phase distortion, are described     

四波混频和级联四波混频效应产生的连续光频率梳

针对产生光频率梳需要锁模激光种子源的难题,创新性提出了用连续光源产生光频率梳,应用色散平坦高非线性光纤解决了宽光谱范围四波混频和级联四波混频效应的相位失配问题。通过实验展示了近40 nm带宽的光频率梳的形成,光频率梳由低功耗,低成本,连续波种子（法布里-珀罗激光器）生成,无需脉冲激光源。连续光频率梳是由在420 m零色散色散平坦高非线性光纤中的四波混频和级联四波混频效应产生的,频谱带宽扩展了近10倍,频率梳的线宽为4.3 MHz。     

Application of optical frequency comb generation with controlled delay circuit for managing the high capacity network system

The generation of optical frequency comb (OFC) with novel controlled delay circuit is presented in this study. The proposed OFC is based on a single laser source which is cascadingly connected with three modulators; all the modulators are tailored by RF signal while incorporating no phase shifter or electrical/optical amplifier. The proposed OFC is used as a source at optical line terminal (OLT) of WDM-PON setup, which serves multiple users with a single laser source making OLT side very cost effective. 41 comb lines with over 40 dB tone to noise ratio and high side mode suppression ratio, least amplitude difference of under 0.3 dB, with cost effective setup is an attractive source for WDM-PON system. The frequency spacing is 32 GHz between OFCG lines which provides enough bandwidth for data transmission, the overall bandwidth provided by this scheme is 1.2 THz. Using DPSK modulation technique across each frequency of OFCG, the controlled delay circuit increases the capacity by factor two, whereas by deploying DQPSK modulation technique, it quadrupled the overall capacity in downlink transmission. Thus, the system offers four times increased capacity ∼1.6 Tbps by deploying state of the art technique for modulation and generating OFC with controlled delay. The average power penalty in the downlink and uplink transmission is 2.5 dBm and 3.13 dBm.     

Generation of orthogonal UWB shaping pulses based on compressed chirp signal

This study investigates a novel method to numerically generate orthogonal ultrawide band (UWB) shaping pulses based on compressed chirp signal. First, a pulse template with less than 1 ns duration time, which is used to construct a Hermitian matrix, is produced with a compressed chirp pulse. Sub-nanosecond orthogonal pulses are then generated for UWB by using the Hermitian matrix eigenvectors. The simulation results show that the power spectral density distribution of the UWB shaping pulses met the constraint of Federal communications commissions (FCC) spectral mask. The shaping pulses not only have higher spectrum utilization ratio and very short time duration but also have excellent autocorrelation and cross-correlation properties, which is an advantage to reduce the interference between multiusers. Especially, a method to produce sub-nanosecond orthogonal UWB shaping pulses by using a relatively longer duration chirp signal is presented.     

All-Optical 40-GHz Time-Domain Fourier Transformation Using XPM With a Dark Parabolic Pulse

We demonstrate a 40-GHz all-optical time-domain Fourier transformation, in which an ideal parabolic phase modulation is applied to a signal by cross-phase modulation (XPM) with a dark parabolic pulse. The dark optical parabolic pulse is generated by the line-by-line spectral manipulation of a 40-GHz picosecond optical pulse source using an arrayed waveguide grating pulse shaper. By applying the parabolic XPM, the time interval for eliminating linear waveform distortions is expanded to a full bit slot. Dispersion slope-induced distortion is successfully eliminated with this scheme.      

Photonic Generation of Chirped Microwave Pulses Using Superimposed Chirped Fiber Bragg Gratings

A novel approach to generating linearly chirped microwave pulses in the optical domain based on spectral shaping and linear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed system, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is spectrum-shaped by an optical filter that consists of two superimposed chirped fiber Bragg gratings (SI-CFBGs) with different chirp rates. The SI-CFBGs form a Fabry-Perot cavity with a cavity length linearly dependent on the resonance wavelength, thus a spectral response with an increased or decreased free spectral range is generated. A chirped microwave pulse with the pulse shape identical to the shaped spectrum is obtained at the output of a high-speed photodetector thanks to the frequency-to-time mapping in a dispersive device. The proposed technique is experimentally demonstrated, a linearly chirped microwave pulse with a central frequency of 15 GHz and a chirp rate of 0.0217 GHz/ps is experimentally generated.     

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     

Prism-based pulse shaper for octave spanning spectra

We demonstrate a prism-based pulse shaping setup that enables the shaping of ultrashort pulses with octave-spanning spectra. In contrast to gratings prisms allow for a high throughput and large bandwidths. The second-order dispersion of the prism material is precompensated by a fused silica prism sequence while higher dispersion orders are compensated by the pulse shaper. This flexible tool for dispersion compensation enables us to generate the shortest pulses ever achieved directly from an oscillator. The resulting pulse is almost transform-limited and shows a clean temporal profile with very low background. We assess the performance and limitations of the setup both experimentally and by theoretical analysis.     

超级信道在高速光标记交换系统中的应用研究

将利用光频梳产生的超级信道引入光标记交换(OLS)系统中。首先,通过仿真实现了能够产生60根可用谱线的光频梳,为光标记和超级信道提供了性能良好的光源;其次,搭建了超级信道光标记交换系统,使用正交频分复用(OFDM)的方法产生超级信道,得到携带1.25Gbit/s光标记的6Tbit/s的超级信道。仿真结果表明:在误码率为10-9时,超级信道各子信道和光标记的接收光功率分别为-5.3和-18.12dBm,实现了性能良好的高速光传输。     

A 750 Mb/s, 12 pJ/b, 6-to-10 GHz CMOS IR-UWB Transmitter With Embedded On-Chip Antenna

This paper presents a novel impulse radio based ultra-wideband transmitter. The transmitter is designed in 0.18 mum CMOS process realizing extremely low complexity and low power. It exploits the 6-to-10 GHz band to generate short duration bi-phase modulated UWB pulses with a center frequency of 8 GHz. No additional RF filtering circuits are required since the pulse generator circuit itself has the functionality of pulse shaping. Generated pulses comply with the FCC spectral emission mask. Measured results show that the transmitter consumes 12 pJ/b to achieve a maximum pulse repetition rate of 750 Mb/s. An optional embedded on-chip antenna and a power amplifier operating in 6-10 GHz band are also designed and investigated as a future low cost solution for very short distance IR-UWB communications.     

达曼类滤波器的飞秒脉冲整形技术

为了研究飞秒激光时空变换整形技术,采用理论分析与计算机模拟结合讨论的方法,分析了以达曼类滤波器为模板的4f 系统的飞秒脉冲整形技术。以梯度算法优化设计出达曼类滤波器,包括等间距、等强度和不等间距、不等强度达曼光栅,并模拟讨论了产生的超快时域多脉冲与光谱平面上调制周期的重复数、元件间距不等效应和波长效应的影响。结果表明,以达曼类滤波器为模板的飞秒脉冲整形中,模板周期重复数越多,输出脉冲的质量越高;元件间距不等和波长效应使输出脉冲的平均度和衍射效率都有所降低。