Femtosecond enhancement cavity with kilowatt average power

Femtosecond enhancement cavity(fsEC) has been proved to be a powerful tool in a diverse range of applications.Here, we report the recent progresses in building an fsEC on kilowatt level average power, with the aim of realization of intracavity high harmonic generation(HHG) and extension of the wavelength of femtosecond optical frequency comb from infrared(IR) to extreme ultraviolet(XUV). Upon mode-matching optimization and cavity length locking, an intracavity average power of 6.08 kW is reached and the corresponding buildup is 225. After introducing noble gas of Xe into the focus region, clear sign of plasma has been observed. The generated HHG is also coupled out by a sapphire plate placed at Brewster's angle for the fundamental laser. Our work paves the way for the realization of an XUV comb.     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

Production of narrowband tunable extreme-ultraviolet radiation by noncollinear resonance-enhanced four-wave mixing

Fourier-transform-limited extreme-ultraviolet (XUV) radiation (bandwidth approximately < 300 MHz) tunable around 91 nm is produced by use of two-photon resonance-enhanced four-wave mixing on the Kr resonance at 94 093 cm(-1). Noncollinear phase matching ensures the generation of an XUV sum frequency 2 omega1 + omega2 that can be filtered from auxiliary laser beams and harmonics by an adjustable slit. Application of the generated XUV light is demonstrated in spectroscopic investigations of highly excited states in H2 and N2.     

Optical generation of a precise microwave frequency comb by harmonic frequency locking

A semiconductor laser under negative optoelectronic feedback is applied to the generation of a microwave frequency comb through the nonlinear dynamics. The laser system is operated in a harmonic frequency-locked pulsing state, where its power spectrum is a microwave frequency comb that consists of multiples of a locking frequency. Every frequency component of the comb can be simultaneously stabilized by simply injecting an external microwave modulation at any component of the comb. This phenomenon can be viewed as a kind of microwave injection locking of the laser dynamics.     

Optical clockwork with an offset-free difference-frequency comb: accuracy of sum- and difference-frequency generation

We demonstrate a simple optical clockwork mechanism based on the broadened frequency comb of a femtosecond laser and on difference-frequency generation (DFG) in a nonlinear crystal. The DFG comb possesses a vanishing carrier envelope offset frequency that permits the construction of a simple and thus potentially more stable optical clockwork. In addition it offers the possibility of extending the frequency comb into the infrared spectral region. The overall accuracy and stability of the DFG comb relative to the initial frequency comb were measured to be 6.6 x 10(-21) and 10(-18) tau(-1), respectively, where tau is the averaging time in seconds. Assuming that sum- and difference-frequency generation are independent processes, our measurements suggest a <10(-20) accuracy for them.     

Octave spanning tunable frequency comb from a microresonator

We report the generation of an octave-spanning optical frequency comb in a continuous wave laser pumped microresonator. The generated comb spectrum covers the wavelength range from 990 to 2170 nm without relying on additional external broadening. Continuous tunability of the generated frequency comb over more than an entire free spectral range is demonstrated. Moreover, the linewidth of individual optical comb components and its relation to the pump laser phase noise is studied. The ability to derive octave-spanning spectra from microresonator comb generators represents a key step towards f-2f self-referencing of microresonator-based optical frequency combs.     

Photonic generation of high quality frequency-tunable millimeter wave and terahertz wave

A scheme for the photonic generation of frequency-tunable millimeter wave and terahertz wave signals based on a highly flat optical frequency comb is proposed and demonstrated experimentally.The frequency comb is generated using two cascaded phase modulators(PMs)and an electro-absorption modulator(EAM).The frequency comb covers a 440-GHz frequency range,with 40-GHz comb spacing and less than 2-dB amplitude variation.By filtering out two of the comb lines with 50 dB out of the band suppression ratio,high frequency-purity and low phase noise millimeter wave or terahertz wave signals are successfully generated,with frequencies ranging from 40 to 440 GHz.     

Optical frequency comb generation using a quasi-velocity-matched Fabry-Perot phase modulator

We report about the optical frequency comb generation by means of a bulk type phase modulator with periodic domain inversion. The phase modulator operates as a quasi-velocity-matched electro-optical phase modulator having high modulation efficiency. It is possible to generate a broadband optical frequency comb by using the modulator installed into an Fabry-Perot resonator. In this experiment, we made the quasi-velocity-matched phase modulator and then used it to generate the efficient optical frequency comb. From this result, we could confirm operation characteristics of the optical frequency comb and the utility of the phase modulator respectively.     

Full stabilization of a microresonator-based optical frequency comb

We demonstrate control and stabilization of an optical frequency comb generated by four-wave mixing in a monolithic microresonator with a mode spacing in the microwave regime (86 GHz). The comb parameters (mode spacing and offset frequency) are controlled via the power and the frequency of the pump laser, which constitutes one of the comb modes. Furthermore, generation of a microwave beat note at the comb's mode spacing frequency is demonstrated, enabling direct stabilization to a microwave frequency standard.     

Generation of near-infrared frequency combs from a MgF₂ whispering gallery mode resonator

We report on generation of a 20 nm wide, 35 GHz repetition rate optical frequency comb in a magnesium fluoride whispering gallery mode resonator pumped with 2 mW of 1543 nm light. The high efficiency of comb generation is associated with the small anomalous group velocity dispersion of the resonator. Growth dynamics of the comb is studied and compared with earlier theoretical predictions.     

Generation of a reference frequency comb by cascaded four-wave mixing enhanced by Raman amplification

The generation of standard reference frequencies close to the ITU channels is essential for the frequency calibration of DWDM systems. This paper describes the generation of a reference frequency comb based on the combination of a cascaded four-wave mixing in two semiconductor optical amplifiers and Raman amplification in a dispersion-shifted optical fiber. As a result we have achieved a stable frequency comb with 36 reference lines separated by a constant frequency spacing of 177 GHz. The seed of the comb is combination of two narrow-linewidth semiconductor lasers which are locked to two absorption lines of the acetylene (¹²C₂H₂).     

利用远紫外光产生多重谐波截止能量延伸

数值研究了氦离子在红外激光场与远紫外激光场驱动下辐射高次谐波的过程.计算结果表明,当远紫外激光场加入后,电子在与母核回碰的过程中会再吸收远紫外光子,进而导致谐波截止能量以远紫外光子能量为间隔的延伸.随着远紫外激光场场强的增强,电子在回碰过程中会吸收更多的远紫外光子,因此多重谐波截止能量会进一步延伸.最后,随着红外激光场与远紫外激光场延迟时间的改变,多重谐波截止能量的延伸可以得到调控.     

Generation of Continuum Extreme-Ultraviolet Radiation by Carrier-Envelope-Phase-Stabilized 5-fs Laser Pulses

Coherent extreme-ultraviolet （XUV） radiation is studied by interaction of carrier-envelope （CE） phase stabilized high energy 5-fs infrared （800 nm） laser pulses with neon gas at a repetition rate of 1 kHz. A broadband continuum XUV spectrum in the cut-off region is demonstrated when the CE phase is shifted to about zero, rather than modulated spectral harmonics when setting of CE phase is nonzero. The results show the generation of isolated attosecond XUV pulses.     

Short-pulse high-intensity excimer lasers — A powerful tool for the generation of coherent VUV and XUV radiation

Different approaches for the generation of coherent VUV and XUV radiation with a 400 fs KrF excimer-laser system are studied. In nonlinear optical experiments it is shown that four-wave difference-frequency mixing in Xe, using a near two-photon resonance with the KrF laser radiation, is well suited for the generation of tunable VUV radiation in the range 130–200 nm. Conversion efficiencies of 2% and output energies up to 260 J have been demonstrated. Further prospects to achieve J energies are discussed. Using this VUV source and the KrF laser, powerful XUV radiation can be generated by different low-order frequency mixing processes. In first experiments on this subject, direct frequency tripling of the KrF laser pulse has resulted in 14 J XUV radiation at 83 nm.For the realization of soft-X-ray lasers, specific advantages of short-pulse KrF drivers are discussed. Novel scenarios based on a hybrid KrF/Ti: sapphire laser system and multiphoton resonant excitation are considered.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Modulation of multiphoton resonant high-order harmonic spectra driven by two frequency-comb fields

The frequency-comb structure in the extreme ultraviolet(XUV) and vacuum ultraviolet(VUV) regions can be realized by the high-order harmonic generation(HHG) process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental-and third-harmonics, respectively.The many-mode Floquet theorem(MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamentaland third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts(CEPs) fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields.We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation(HHG) spectra,driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths.It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.     

Optoelectronic oscillator using a LiNbO3 phase modulator for self-oscillating frequency comb generation

Optical frequency comb generation by using a novel optoelectronic oscillator (OEO) is proposed and demonstrated with the emphasis placed on self-oscillating operation. In the OEO, a wideband LiNbO3 phase modulator is driven with a large-amplitude radio-frequency (RF) feedback signal to generate a deeply phase-modulated light wave; accordingly, an optical frequency comb with a bandwidth greater than the RF signal is generated by self-oscillation. Although it generates multifrequency components, the OEO exhibits characteristics of a single-mode oscillator. Its operation is stable and self-starting. An optical frequency comb with a 120 GHz bandwidth and 9.95 GHz frequency spacing was successfully generated by self-oscillation at a single frequency.     

Generation of 10- microJ coherent extreme-ultraviolet light by use of high-order harmonics

We demonstrate the generation of 10-microJ coherent extreme-ultraviolet (XUV) light at wavelengths from 73.6 to 42.6 nm, using high-order harmonics. The peak power of this coherent XUV light is estimated to be 0.13 GW at 62.3 nm, and the peak brightness achieved was 3x10(28) photons/(mm(2)mrad(2)s) . To our knowledge, this XUV energy is the highest value achieved with high-order harmonics.     

Demonstration of a HeNe/CH4-based optical molecular clock

We implement a simple optical clock based on the F2(2) [P(7), v3] optical transition in methane. A single femtosecond laser's frequency comb undergoes difference frequency generation to provide an IR comb at 3.39 microm with a null carrier-envelope offset. This IR comb provides a phase-coherent link between the 88-THz optical reference and the rf repetition rate. Comparison of the repetition rate signal with a second femtosecond comb stabilized to molecular iodine shows an instability of 1.2 x 10(-13) at 1 s, limited by microwave detection of the repetition rates. The single-sideband phase noise of the microwave signal, normalized to a carrier frequency of 1 GHz, is below -93 dBc/Hz at 1-Hz offset.     

Widely-tunable mid-infrared frequency comb source based on difference frequency generation

We report on a mid-IR frequency comb source of unprecedented tunability covering the entire 3-10 μm molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman-shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at the 4.7 μm wavelength.     

PPLN晶体差频测量飞秒激光脉冲的载波包络相移

在飞秒激光频率梳系统中，通常采用自参考技术测量飞秒激光脉冲的载波包络相移，但该技术需要采用光子晶体光纤进行光谱扩展从而增加了系统的不稳定性，这种技术已经制约了高稳定度的飞秒激光频率梳的发展.采用PPLN晶体差频法测量了宽谱钛宝石振荡器输出的7fs激光脉冲的载波包络频移，得到了大于30dB的拍频信号，为研制无光纤的新一代高稳定度光学频率梳奠定了基础.