Generation of Power-Efficient FCC-Compliant UWB Waveforms Using FBGs: Analysis and Experiment

In this paper, we design, analyze, and demonstrate experimentally U.S. Federal Communications Commission (FCC)- compliant power-efficient ultrawideband (UWB) waveforms generated by optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs the frequency-to-time conversion. The waveform is inscribed in the frequency domain by the fiber Bragg grating (FBG). A significant challenge for this approach is elimination of an unwanted, positive rectangular pulse superimposed on the desired waveform. Our innovative use of balanced photodetection eliminates this pedestal, assuring compliance with the FCC mask at low frequency. Three UWB pulses with duration of 0.3,0.6, and 1.2 ns are designed and tested experimentally. Whereas an excellent match between the optimized and measured pulses is achieved for the simpler, shorter duration waveforms, the noise in the fabrication process of FBGs limits the generation of the more complex, longer duration waveforms.     

铜蒸气激光振荡器与放大器中的黄绿光脉冲及其对R6G染料激光输出效率的影响

本文从实验上研究了铜蒸气激光振荡器从振荡到稳定输出过程中黄光脉冲(578.2nm)与绿光脉冲(510.6nm)的强度变化、振荡放大系统中触发延时对放大器两种激光脉冲的强度和相对延迟的作用,并指出R6G染料激光的输出效率不但与泵浦光—铜蒸气激光的黄绿光强度比的大小有关,并且还与这两种光脉冲的相对延迟的大小有明显关系.     

基于氢气填充空芯光子晶体光纤的全光纤型气体拉曼光源特性

理论和实验研究了调Q光纤激光脉冲抽运基于氢气填充空芯光子晶体光纤气体腔的全光纤型气体拉曼光源的特性。抽运光脉冲波长为1064.7nm时,产生的Stokes频移光波长为1135.7nm。理论和实验结果均表明,产生的Stokes频移光脉冲宽度远小于抽运光脉冲,并且,Stokes频移光脉冲宽度随抽运光脉冲能量的提升而增加。此外,减小抽运光脉冲宽度,可以降低拉曼阈值抽运能量、提高Stokes频移光的转换效率。在重复频率为5kHz、脉冲宽度为125ns的调Q光纤激光脉冲抽运下,实验测得拉曼阈值抽运能量和拉曼阈值点处转换效率分别为2.13μJ和9.82%。     

Optical Spatial Quantization for Higher Performance Analog-to-Digital Conversion

A novel optical spatial quantized analog-to-digital conversion scheme for real-time conversion at ultrahigh sampling frequencies is presented. At each sampling instant, the analog input voltage deflects an optical sampling pulse onto an array of photodetectors. The output code is derived from the output voltages of the photodetectors on which the optical beam lands. Particular benefits of the proposed architecture are significant reduction in jitter through the use of a mode-locked laser to generate the sampling pulses, high quantization bandwidth through a fully optical quantization scheme, and the system simplicity through the use of just one phase modulator and an embedded binary encoder in the binary-connected photodetector arrays. We experimentally demonstrate an eight-level quantization consuming only 7.2 pJ per quantization with 18-GHz bandwidth, projected to an estimated bandwidth of 30 GHz. Measured 8-ps full-width half-maximum photodetector output voltages promise the potential of realizing a 3-bit 125-GS/s analog-to-digital converter.      

All-optical wavelength conversion of short pulses and NRZ signalsbased on a nonlinear optical loop mirror

Wavelength conversion of short pulses at 10 GHz based on a nonlinear optical loop mirror (NOLM) is experimentally and numerically investigated for the case of small group velocity dispersion and walkoff between the control pulses and continuous lightwaves. Experimental and numerical simulation results show that the pulsewidths of the converted signals at different wavelengths are almost the same, and the pulsewidths are compressed when the peak power of the control pulse is smaller than a certain value. An RZ optical source containing eight wavelengths having a high sidemode suppression ratio, equal amplitudes and almost the same pulsewidths is obtained by using wavelength conversion in a NOLM consisting of a common dispersion shifted fiber. 10 Gb/s NRZ wavelength conversion based on the NOLM is demonstrated for the first time and certain conclusions in some of the references are confirmed by our experimental results     

准单模光纤中受激喇曼散射(SRS)的时间特性和光谱特性

本文报道了在准单模石英光纤中对于ps泵浦脉冲,研究了多级SRS脉冲、泵浦光脉冲的时间特性和光谱特性以及它们之间的时间分离。发现SRS转换效率很高(80％以上),并且SRS的产生位置距光纤输入端很近(1.5m),对于63m光纤,观察到几乎连续的9级SRS光谱     

Microcavity-enhanced surface-emitted second-harmonic generation forultrafast all-optical signal processing

By incorporating an integrated microcavity into an optical waveguide structure with vertical quasi-phase-matching, we have realized surface-emitted second-harmonic generation devices that significantly enhance the conversion efficiency for optical pulses in the picosecond and sub-picosecond regimes. We demonstrate both theoretically and experimentally that nonlinear interactions involving short optical pulses can be enhanced by a microcavity, even when the resonance width is substantially narrower than the spectral content of the pulse. The resulting enhancement enables practical signal processing functions such as ultrafast optical time-division demultiplexing at 1.55 μm in multilayer AlGaAs structures     

Logic Functions, Devices, and Circuits Based on Parametric Nonlinear Processes

Second-harmonic generation and parametric down-conversion processes have been studied as the basis of all-optical logic gates. All possibilities that are obtainable with both the low and high conversion efficiencies of such processes have been analyzed here. XOR and AND gates are also experimentally proven by using 1-ps pulses at 800 nm within a beta-BaB₂O₄ crystal, reaching conversion efficiencies of as high as 80%. Based on these phenomena, complex algebraic operations are proposed for performing several different logic functionalities particularly concerning network switching and arithmetic calculation.     

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.     

Efficient temporal compression of coherent nanosecond pulses in acompact SBS generator-amplifier setup

A pulse compressor based on stimulated Brillouin scattering (SBS) in liquids is experimentally and theoretically investigated. It allows for the compression of Fourier-transform limited nanosecond pulses of several hundreds of millijoules of energy with both high conversion efficiency and a high temporal compression factor. The two-cell generator-amplifier arrangement is of a compact design not requiring external attenuation of the generator cell input energy. Pulses from an injection-seeded, frequency-doubled Nd:YAG laser of 300-mJ energy were compressed by a factor variable between 6 and 21 at up to 75% reflectivity. Deviation from unity SBS reflectivity is predominantly determined by optical component losses. The generation of 270-ps pulses with high beam quality was achieved in liquid methanol. These powerful pulses of variable duration are difficult to produce with common laser systems and are highly suited for the generation of high-harmonics in gases     

Spectro-temporal imaging of femtosecond events

We demonstrate a new technique for the direct, real-time, femtosecond scale temporal measurement based on the conversion of temporal information to the spectral domain. The potential of the method has been experimentally investigated with an optical fiber spectral compressor device, first stretching and up-chirping the pulses in a prism dispersive delay line, and afterwards compensating the induced chirp by means of cross-phase modulation in a single-mode fiber. Spectro-temporal imaging (STI) reduces the problem of high-resolution temporal measurements to standard spectrometry     

A coherent dual-frequency phase-sensitive reflectometer with amplitude modulation of probing pulses

The results of investigation of a new type of the optical-fiber coherent phase-sensitive reflectometer with two different optical frequencies of probing pulses are presented. The use of pulses with different optical frequencies allows recovering of the signal of an external action on the reflectometer optical fiber by means of phase diversity technique. The applicability of the proposed technique is justified theoretically and experimentally.     

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.     

Wavefront reconstruction (conjugation) by stimulated scatterings: A review

In this paper, we give review of our work on the holographic image reconstruction of the volume object, illuminated by giant laser pulses of nano- and picosecond duration, and of its Fourier spectrum which has been observed experimentally in Raman, Brillouin, and Rayleigh stimulated scatterings. Image reconstruction laws have been investigated. Wavefront conjugation with transformation, obeying holography principles, has been shown to take place at the backward SRS. A physical model of the phenomenon is given. The diffraction efficiency of dynamic holograms in the stimulated scatterings has been measured. The possibility of the light beams spatial structure conversion at the stimulated scatterings has been experimentally shown.     

The effect of pulse walkoff on stimulated Raman scattering in fibers

We have experimentally investigated stimulated Raman scattering in single-mode fibers in the regime of large Raman Stokes pulse walkoff from the pump pulse by the effect of group-velocity dispersion. Measurements are made with 36 ps duration pulses at 532 nm produced from a frequency-doubled and harmonically mode-locked Nd:YAG laser. We find that for 20 percent conversion, the Raman output is produced about two walkoff lengths into the fiber as a pulse of approximately the same duration as the input pulse. We also find that the Raman pulse is produced with a strong frequency chirp.     

基于超连续谱产生的波长转换实验研究

本文对基于超连续谱产生的波长转换进行了实验研究。实验中首先搭建了主动锁模掺铒光纤激光器,然后将其产生的脉冲放大后注入色散位移光纤,利用双通结构获得了超连续光谱,最后用光纤光栅滤波实现了波长转换。实验得到了宽度为80nm的超连续谱,用光纤光栅滤波得到了峰值功率为-37.27dBm,信噪比是26.83dB,脉冲宽度为3ns的转换脉冲。     

基于光子晶体光纤飞秒激光技术的高功率紫外激光源

实验研究了一种基于大模场面积光子晶体光纤飞秒激光技术的紫外飞秒激光源.分析了群速失配下的倍频光和基频光的走离长度,并实验比较了不同长度的BBO晶体的倍频功率和效率.分别采用5 mm和0.18 mm的两块BBO晶体,在Ⅰ类相位匹配条件下,对光子晶体光纤放大器输出的脉宽为110 fs,重复频率50 MHz的1040 nm飞秒激光进行腔外二倍频(SHG)和四倍频(FHG),获得了高功率紫外飞秒激光.在20 W的平均功率抽运下,获得了8.88 W的二倍频绿光输出,转换效率为44.4%.同时获得了656 mW的四倍频260 nm紫外激光,单脉冲能量13 nJ,最高功率时二次谐波(SH)到四次谐波(FH)的转换效率为7.39%.     

Calculated and experimental response of thin cylindrical antennas to pulse excitation

We have computed the response of a thin, cylindrical monopole antenna to dc pulse excitation, both in transmission and reception, by use of Fourier transforms and the complex impedance and complex effective height, for pulses of space length ranging from two tenths to twice the monopole length. We have also experimentally determined the reponse and have obtained similar results. We conclude from this comparison that present steady-state antenna theory together with Fourier methods will yield reliable results in calculating the response of cylindrical antennas to dc pulses. We have also made certain other measurements of response to pulses and step functions which are reported herein.     

All-optical modulation and demultiplexing systems with significanttiming jitter tolerance through incorporation of pulse-shaping fiberBragg gratings

In this letter, we demonstrate the use of a superstructured fiber Bragg grating to preshape optical pulses to obtain optimal operation of nonlinear all-optical switches. Specifically, we demonstrate the conversion of 2.5-ps soliton pulses into 20-ps rectangular pulses at the input to both fiber and semiconductor optical amplifier-based switches, and show that rectangular switching windows can be achieved thereby providing a 5-10-fold reduction in timing jitter sensitivity. Error free penalty free optical time-division-multiplexing switching was readily achieved over a ±7-ps timing mismatch range for the square pulse driven fiber nonlinear optical loop mirror switch versus a ±1-ps range for the switch driven directly with 2.5-ps laser pulses     

Generation of Arbitrary UWB Waveforms by Spectral Pulse Shaping and Thermally-Controlled Apodized FBGs

We propose and experimentally demonstrate an arbitrary UWB pulse generator. The proposed technique is based on spectral pulse shaping and frequency-to-time conversion. The reconfigurability of this technique comes from changing the apodizaton of a chirped fiber Bragg grating (FBG) using a series of heating elements (HE). By setting the appropriate temperature set to the HEs, any predesigned UWB waveforms can be generated with high precision. The effective isotropically radiated power (EIRP)-optimized and U.S. Federal Communications Commission (FCC)-optimized pulses as well as the traditional Gaussian monocycle and doublet UWB waveforms are generated which are in excellent match with the designed target pulse shapes. While other arbitrary pulse generators have used similar strategies (spectral shaping and frequency-to-time conversion), ours uses inexpensive technologies with the potential for practical, compact packaging.