Higher harmonic generation in CMOS/SOS ring oscillators

Harmonic generation is found with 101-stage ring oscillators. Harmonics have not been observed for the usual ring oscillators with a small number of stages. If one mistakes the higher harmonic generation for the fundamental, he obtains a wrong propagation delay which is shorter than the real one. It is shown experimentally and theoretically that only odd harmonics are generated for the ring oscillators with an odd number of stages. The propagation delay tpdof the nth harmonic oscillation is given byt_{pd} = n cdot T/2NwhereTis the observed repetition period andNthe number of stages. Computer simulation shows that a ring oscillator with an even number of stages can also oscillate if every inverter is the same, and that the oscillation decays if there is asymmetry in the inverter chain. IfNis large and the effects of the deviations of the transistor parameters cancel one another, the harmonic oscillation that happens to be generated can continue.     

对称棱镜式环形谐振腔结构优化

由直角梯形棱镜构成的环形谐振腔的输出信号在机械振动环境下会受到影响,使输出波形幅值产生调制。为了改善谐振腔输出信号质量,从工程探索的角度出发,首次将对称型全反射棱镜纳入谐振腔的结构设计方案中。通过理论计算和有限元分析的方法,对比了两种谐振腔的光线传输特性和应力分布。结果表明,对于传输波长为632.8 nm的激光谐振腔而言,对称型结构可以保证构成谐振腔的光学元件具有对称的应力分布,降低棱镜上的应力双折射效应,提高输出信号的稳定性。对称性棱镜环形谐振腔的激光传输轨迹更稳定,装配要求更宽松,应力极值仅为非对称结构的15%。通过对比,在一个激励周期内,对称型谐振腔的激光输出能量波动相比原始结构降低了52.63%,平稳性获得了显著的改善。使用对称型环形谐振腔,对于提高我国现阶段环形激光传感器件的输出稳定性提供了一种新的技术途径。     

Bandwidth-limited picosecond pulse generation by hybrid mode-locking in a ring cavity GaAlAs laser

Bandwidth-limited picosecond pulses have been obtained using hybrid active/passive mode-locking in an external cavity semiconductor diode ring laser. Active mode-locking by application of sinusoidal RF modulation at the cavity round-trip frequency to a diode laser providing gain is supplemented by passive mode-locking in a second laser which is biased to provide saturable loss. Simultaneous streak camera and scanning Fabry-Perot interferometer measurements show that bandwidth-limited pulses of 25 Ps duration are obtained. Operation is not limited to near laser threshold, and frequency detuning sensitivity is improved when compared to the case of purely active mode-locking.     

腔内四倍频激光增强反射腔设计

腔内反射增强技术可有效提高紫外激光四倍频效率,但反射增强腔存在多次反射光斑不重合、光束发散角不一致的问题。为了获得更好的光束质量,设计了一种实现多次反射绿光光束可重现的反射腔结构,采用传输矩阵法计算了反射光束及四倍频转换等参量。结果表明,可重现的反射腔不仅能有效避免反射增强腔内光束半径不一致导致的光束质量差的问题,还可进一步提升四倍频效率。     

二次谐波产生的光学倍频腔自锁定系统

二次谐波产生技术能有效扩展现有的激光波段,被广泛应用于原子分子物理、光物理、生物医学等多个领域。倍频腔的长期锁定是倍频技术中的一项关键技术,它直接关系到输出激光的功率稳定性和可靠性,目前的商用二次谐波产生激光器都会存在不同程度上的失锁状态。提出了一种基于积分扫描来实现连续激光倍频器自锁定的方案,能在没有人为调节的情况下连续工作,不需要额外的功率监测和扫描信号,失锁后可自动恢复锁定,最终实现了长期稳定的二次谐波产生激光输出。     

Short-pulse generation with broad-band tunability fromsemiconductor lasers in an external ring cavity

Short optical pulses are generated by actively mode locking semiconductor lasers in an external ring cavity with a very broad tuning range from 795 to 857 nm. The wide tunability is possible because the gain bandwidth is broadened by the use of asymmetric dual quantum wells for the semiconductor laser material. Assuming a Gaussian shape, the generated pulses have pulsewidths of 13-21 ps and spectral widths of 24.5 Å for the tuning range. The mode-locked spectrum contains almost no amplified spontaneous emission noise     

Enhancement of free-electron coherent harmonic generation by use ofthe dispersive function on a storage ring

An alternative to the free-electron laser (FEL) oscillator at short wavelengths (λ<2000 Å) is the coherent harmonic generation (CHG) process; the electron beam, when passing through the undulator, acts as a nonlinear medium and allows one to obtain coherent harmonics of an external laser. However, this coherent emission is rather weak, particularly on the high-order harmonics (n⩾5). It is shown that the introduction of an energy dispersive function on a storage ring decreases the local energy spread of the beam and then increases the CHG by several orders of magnitude     

Optical pulse shaping by cross-phase modulation in a harmonic mode-locked semiconductor fiber ring laser under large cavity detuning

The authors experimentally investigate the phenomena of the pulse-shape distortion due to the drift of cavity length or modulation frequency in a harmonically mode-locked semiconductor fiber ring laser (SFRL). Also, it is experimentally demonstrated that the optical pulse shaping by cross-phase modulation (XPM) can be applied to enhance the robustness of a harmonic mode-locked semiconductor ring laser to the environmental factors, e.g., the modulation-frequency drift and/or the cavity-length drift. The mechanism of pulse-shape distortion due to the modulation-frequency drift or cavity-length drift is explained and experimentally verified. As the locking condition of the SFRL goes to the rational harmonic mode-locking (RHML) regime, the pulse-shape distortion becomes dominant. To validate the explanation, a simple equation for estimating and comparing the amount of cavity-length detuning in terms of frequency is proposed for comparison, which is called the equivalent frequency detuning (EFD). The distortion mostly appeared in the trailing edge of the pulse. By selectively filtering the XPM-induced red-chirped wavelength components in the rising edge of the pulse, the mode-locked pulse train with large pulse-shape distortion can be reshaped to regenerate a mode-locked pulse train with less pulse-shape distortion than the pulse train without the proposed scheme. This method can be used to reduce the pulse-shape distortion effectively, maintaining the pulse repetition rate under the large modulation-frequency detuning, or equivalently, large cavity-length detuning. The proposed scheme is able to generate the pulse-trains robust to the modulation-frequency drift and/or cavity-length drift of the mode-locked SFRL.     

Microwave harmonic generation in a plasma capacitor

Microwave harmonic generation in a plasma capacitor is investigated both theoretically and experimentally. Deficiencies of previous harmonic generation theories are pointed out and a nonlinear one-dimensional mathematical model, which includes the reactive nonlinearities due to the spatial variation of E, is developed. This nonlinear plasma capacitor model indicates that under certain electron-density and capacitor-plate spacing conditions strong harmonic resonances and an antiresonance are present. The antiresonance occurs when nω= ωpand the resonances occur when nω≳ ωp, where ωp=plasma frequency, ω= the fundamental frequency, and n is the harmonic number. Harmonic generation in a number of gases is investigated experimentally in a coaxial discharge structure. Third-harmonic (9-GHz) efficiencies of up to 13 percent, and third-harmonic output power in excess of 600 mW are reported. Double-probe and microwave-scattering measurements show that enhanced harmonic generation occurs at the above resonances and, hence, substantiate the non-linear plasma capacitor theory. In order to explain the harmonic output power variations that occur with pressure variation and gas type, a graphical method of analysis based on experimental fact and pressure-collision frequency-electron energy curves is presented. This method of analysis not only yields results that compare qualitatively with experimental observations, but it may also possibly be used to design and predict the performance of future plasma frequency multipliers.     

谐振式微光学陀螺仪谐振微腔的最新进展

基于Sagnac效应的谐振式微光学陀螺(Resonant Micro-Optical Gyros, RMOG)在集成化、小型化和灵敏度等方面具有巨大潜力,在微纳卫星姿态控制、机器人控制、医学诊断和检测仪器等领域中具有广阔的应用前景,成为近年来研究的热点。谐振微腔作为谐振式微光学陀螺的核心敏感元件,其光学特性与陀螺系统的性能息息相关,谐振腔的研究进展已经严重制约到谐振式微光学陀螺的发展,目前可以通过集成光学技术、微纳光学加工技术和新型材料的应用来减小谐振腔的重量和尺寸,降低成本和功耗,增加系统的可靠性和性能指标。结合期刊会议和相关研究机构披露的信息,简要介绍了谐振式微光学陀螺的发展现状、基本原理以及谐振微腔的特征参量,列举了近期国内外谐振微腔的各种新型结构设计并分析了不同结构的特点与潜力,此外还综述了近期国内外制作谐振微腔的新型材料并总结了不同材料的光学特性,初步探讨了谐振式微光学陀螺敏感单元谐振微腔的后续发展方向和技术发展途径。     

Reply to comments on "Higher harmonic generation in CMOS/SOS ring oscillators"

Arguments presented in the referenced paper [1] imply that higher order modes of oscillation should not be expected in ring oscillators with a small number of stages. In contrast to this, third and fifth harmonic oscillations have been observed in 15-stage Si MESFET ring oscillators using BFL inverters. It is explained that the decay mechanism presented in the referenced paper applies primarily to even harmonics in ring oscillators with an even number of stages. Higher order modes of oscillation can be sustained in ring oscillators with a relatively small odd number of stages if the rise and fall times of the inverters used are short in comparison with their total delay.     

Electromagnetic mode analysis on photonic-band-gap resonant cavity in a gyrotron

The traditional cylinder resonant cavity in a gyrotron is replaced with a photonic-band-gap resonant cavity (PBGC) with a structure of triangular arrays of metal posts, and the cold character of the TE operating modes in the PBGC are analysed and calculated. Under the consideration of the cavity structure features, a research method for the mode theory on PBGC is provided, single-mode operation in the cavity is found, and the distribution characteristics of the azimuthally inhomogeneous electromagnetic modes are reported. The study provides a physical theory fundament which reveals possibilities for changing the essential operating status of a gyrotron.     

Perturbation of a resonant cavity by a small bianisotropic sphere

Perturbation of a resonant cavity by the introduction of a small bianisotropic sphere in its interior is analyzed. It is shown that the resonance frequency can be altered merely by rotating the bianisotropic sphere at a fixed location.     

使用非均匀特异媒质覆层的高增益谐振腔天线

采用非均匀特异媒质覆层,设计了一种具有高增益特性的新型谐振腔天线。该谐振腔天线采用矩形微带贴片天线作为辐射单元,安装在金属谐振腔内,其谐振频率为10 GHz。为提高天线增益,将腔表面安装蚀刻在微带基板上,由周期性单元组成特异媒质覆层。迥异于常规的单元尺寸均匀一致特异媒质覆层,本文研制的非均匀特异媒质覆层包含9×9个矩形单元,单元大小渐变。仿真表明,与常规均匀特异媒质覆层相比,该新颖的非均匀特异媒质覆层相当大程度提升了天线定向辐射性能：天线增益提高1.2 dB(从20.3 dBi增加到21.5 dBi),天线的旁瓣得到了抑制,主旁瓣比下降5 dB;同时,天线谐振频率和阻抗带宽等其他性能基本保持不变。     

Measurement of material properties with a tunable resonant cavity

A tunable resonant cavity is used to measure the complex permittivity of dielectrics and the surface conductivity of metals with high precision. The analytical approximations, traditionally used in the cavity perturbation techniques, have been replaced with a rigorous numerical solution of the electromagnetic field distribution inside the rotationally symmetric cavity filled with inhomogeneous dielectrics. The measured values of the unloaded Q factor are compared with the numerically computed values over the tuning range. The least-squares data fitting procedure yields simultaneously the values of the loss tangent and the surface conductivity, and their standard deviations.<>     

High-order harmonic generation in plasmas

The phenomenon of harmonic generation by electrons oscillating in high-intensity laser fields is surveyed and assessed as a means of producing short-wavelength radiation. Starting from the seminal early work by Sarachik and Schappert (1970), simple motivatory examples are given of incoherent harmonic generation via nonlinear scattering from single electrons. More recent studies aimed at observing the coherent version of this effect in underdense plasmas are then reviewed and some problems noted in distinguishing these harmonics from those produced via the analogous nonlinear mechanism from bound electrons in rare gases. Finally, the revival of interest in harmonics reflected from overdense plasmas is considered. Short-pulse laser-generated “surface” harmonics appear to offer a very promising, compact, and efficient means of upshifting coherent radiation to sub-10-nm wavelengths     

Enhanced harmonic generation in magnetrons

An X-band rising sun harmonic-generator magnetron has been developed. In order to obtain enhanced second harmonic output power, the magnetron was designed so that the resonant frequency of the second-order π mode was twice the frequency of the first-order π mode. Second harmonic efficiency, the ratio of second harmonic output power to input power, was strongly magnetic field dependent. Large values of second harmonic efficiency and output power occurred for operation in the "valley of efficiency" of the rising sun anode, i.e., for magnetic fields near B=13,000/λ1gauss where λ1is the wavelength of the first-order π mode. Maximum second harmonic efficiency and output power at λ2=1.69 cm were 13 per cent and 46 kw, respectively. For operation at these magnetic fields the ratio of generated second harmonic to fundamental power was of the order unity. This large power ratio can be understood in terms of a phase shift between the fundamental voltage and the current which drives it, together with a modified rotating wave hypothesis where a waving motion, caused by the Γ=0 Hartree component of the azimuthal electric field, is superimposed on the conventional rotation of the space-charge spokes.     

Efficient harmonic generation with a broad-band laser

Harmonic generation using a broadband input beam is studied theoretically and experimentally. Angular spectral dispersion is imposed on the input beam to improve the phase matching and thereby increase the conversion efficiency. The coupled amplitude equations are derived for a grating-dispersed input beam, and it is shown that, in the parameter range of interest, a simple theoretical model based on the intuitive concept of instantaneous frequency suffices to describe the nonlinear interaction. A laser having a frequency-chirped output with a 25-Å bandwidth is used to demonstrate the technique for third-harmonic generation in the small-signal regime