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.     

An ultra-stable referenced interrogation system in the deep ultraviolet for a mercury optical lattice clock

We have developed an ultra-stable source in the deep ultraviolet, suitable to fulfil the interrogation requirements of a future fully-operational lattice clock based on neutral mercury. At the core of the system is a Fabry–Pérot cavity which is highly impervious to temperature and vibrational perturbations. The mirror substrate is made of fused silica in order to exploit the comparatively low thermal noise limits associated with this material. By stabilizing the frequency of a 1062.6 nm Yb-doped fiber laser to the cavity, and including an additional link to LNE-SYRTE’s fountain primary frequency standards via an optical frequency comb, we produce a signal which is both stable at the 10^?15 level in fractional terms and referenced to primary frequency standards. The signal is subsequently amplified and frequency-doubled twice to produce several milliwatts of interrogation signal at 265.6 nm in the deep ultraviolet.     

LD泵浦355nm准连续紫外激光器

报道了一台LD侧面泵浦Nd:YAG晶体的内腔三次谐波转换的全固态准连续紫外激光器。在谐振腔内,1064nm的基频波通过对Ⅱ类相位匹配KTP晶体进行二倍频来产生532nm波长激光,二者再通过对Ⅱ类相位匹配LBO晶体进行和频来获得355nm紫外激光输出。355nm全固态紫外激光器在声光调Q重复频率为2.8kHz下,当输入电流为18A时可得到503mW的激光输出。     

高效率LD端面抽运准连续355 nm激光器

报道了一台激光二极管（LD）端面抽运Nd：YVO₄晶体腔内倍频和腔外和频相结合的声光调Q准连续355 nm 紫外激光器。采用LD端面抽运双侧翼键合YVO₄基质的Nd：YVO₄晶体,在腔内置入Ⅰ类相位匹配的LiB₃O₅（LBO）晶体进行倍频实现1 064 nm和532 nm双波长准连续激光输出,通过消色差透镜将双波长激光聚焦耦合到Ⅱ类相位匹配的LBO 晶体中进行和频,并采用双向和频光路,获得了高效率、高光束质量、高重复频率的准连续355 nm 紫外激光输出。在抽运功率为28.6 W、重复频率为20 kHz时,355 nm激光最大输出功率4.2 W,脉宽为20.6 ns,光-光转换效率为14.7%,激光器光束质量因子M_x²和M_y²分别为1.29和1.23。     

Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm

The absolute frequency of an acetylene-stabilized laser at 1542 nm is measured at its second harmonic (771 nm) by use of a femtosecond optical comb based on a mode-locked Ti:sapphire laser. Frequency stability and reproducibility of the acetylene-stabilized laser are evaluated by the femtosecond comb with a H maser as a frequency reference. The absolute frequency of a laser diode stabilized on the P(16) transition of 13C2H2 is determined to be 194 369 569 383.6(1.3) kHz. The acetylene-stabilized laser serves as an important optical frequency standard for telecommunication applications.     

Generation of femtosecond laser pulses at 263 nm by K_3B_6O_(10)Cl crystal

The third harmonic generation(THG) of a linear cavity Ti:sapphire regenerative amplifier by use of a K_3B_6O_(10)Cl(KBOC) crystal is studied for the first time. Output power up to 5.9 mW is obtained at a central wavelength of 263 nm,corresponding to a conversion efficiency of 4.5% to the second harmonic power. Our results show a tremendous potential for nonlinear frequency conversion into the deep ultraviolet range with the new crystal and the output laser power can be further improved.     

掺Er光纤飞秒激光器中电光晶体对激光器参数的影响

由于受增益介质上能级寿命的影响,掺Er光纤光梳的梳齿线宽一般在百kHz量级.为了实现光梳梳齿线宽的压窄,一种有效的方法是在激光器中增加快速响应的电光晶体,使光纤光梳的伺服锁定带宽提高到百kHz以上,为光纤光梳的快速伺服锁定提供反馈机构.这其中,高品质的飞秒激光器是核心.基于此,本文主要研究了掺Er光纤飞秒激光器中电光晶体对激光器参数的影响.通过计算电光晶体的折射率、色散、相位延迟等参数,分析了电光晶体对激光器参数的影响,并在实验上获得了电光晶体电压对激光器重复频率和载波包络偏移频率的影响,进而通过电光晶体实现了对光纤光梳重复频率和载波包络偏移频率的锁定.通过锁定光纤飞秒激光器与窄线宽激光器的拍频信号,验证了电光晶体的引入使激光器的伺服锁定带宽提高到了236 kHz,为窄线宽飞秒光学频率梳的建立提供了技术基础.     

Narrow linewidth light source for an ultraviolet optical frequency standard

We report an ultra-narrow linewidth light source applicable for a frequency standard in the ultraviolet. The laser is a Nd:YAG laser that emits at 946 nm with 300-mW output power. It is locked to a high-finesse cavity. The minimum Allan deviation is 1.3×10^-14 for an integration time of 1 s, which corresponds to a laser linewidth of 2.8 Hz. The cavity drift is measured by a frequency comb and a single-ion spectrum for different time scales. In order to investigate broadening mechanisms due to the fiber transport and doubling systems, the laser light is frequency doubled with two independent systems and compared. The measured minimum beat-note between the two laser fields is less than 1 Hz. By carrying out a high-resolution scan on a trapped single indium ion, we observe a linewidth of 260 Hz on the ion clock transition. Possible reasons for the broadening effects are discussed. PACS 42.55.Xi; 42.62.Eh; 42.65.Ky     

激光二极管侧面泵浦高功率266 nm紫外激光器

 报道了一种半导体激光列阵侧面泵浦Nd：YAG四倍频266 nm全固态紫外激光器,采用Z型腔结构,Ⅰ类临界相位匹配LBO和BBO晶体分别作为二倍频晶体和四倍频晶体。在调制频率为5 kHz时,最终获得了2.1 W的266 nm紫外激光输出,单脉冲能量420 μJ, 绿光到紫外激光的转换率为13.13%,在相同的泵浦功率下利用V型腔结构仅获得305 mW的266 nm紫外激光输出。     

Widely tunable extreme UV frequency comb generation

Extreme UV (XUV) frequency comb generation in the wavelength range of 51 to 85 nm is reported based on high-order harmonic generation of two consecutive IR frequency comb pulses that were amplified in an optical parametric chirped pulse amplifier. The versatility of the system is demonstrated by recording direct XUV frequency comb excitation signals in He, Ne, and Ar with visibilities of up to 61%.     

Noise properties of an optical frequency comb from a SESAM-mode-locked 1.5-μm solid-state laser stabilized to the 10?13 level

We present a detailed investigation of the noise properties of an optical frequency comb generated from a femtosecond diode-pumped solid-state laser operating in the 1.5-??m spectral region. The stabilization of the passively mode-locked Er:Yb:glass laser oscillator, referred to as ERGO, is achieved using pump power modulation for the control of the carrier envelope offset (CEO) frequency and by adjusting the laser cavity length for the control of the repetition rate. The stability and the noise of the ERGO comb are characterized in free-running and in phase-locked operation by measuring the noise properties of the CEO, of the repetition rate, and of a comb line at 1558?nm. The comb line is analyzed from the heterodyne beat signal with a cavity-stabilized ultra-narrow-linewidth laser using a frequency discriminator. Two different schemes to stabilize the comb to a radio-frequency (RF) reference are compared. The comb properties (phase noise, frequency stability) are limited in both cases by the RF oscillator used to stabilize the repetition rate, while the contribution of the CEO is negligible at all Fourier frequencies, as a consequence of the low-noise characteristics of the CEO-beat. A?linewidth of ??150?kHz and a fractional frequency instability of 4.2×10^?13 at 1?s are obtained for an optical comb line at 1558?nm. Improved performance is obtained by stabilizing the comb to an optical reference, which is a cavity-stabilized ultra-narrow linewidth laser at 1558?nm. The fractional frequency stability of 8×10^?14 at 1?s, measured in preliminary experiments, is limited by the reference oscillator used in the frequency comparison.     

高效全固化紫外四倍频激光器的研究

用半导体激光器抽运的调Q YLF倍频激光器抽运掺钛蓝宝石激光器，用LBO晶体在腔内产生二次谐波，再聚焦到BBO晶体上产生四次谐波深紫外激光。为了提高谐波转换效率，在腔内分别插入了单片、组合双折射滤光片和聚焦透镜，在平均功率为3.6W，波长为527nm的抽运光下得到410mW，417nm的蓝光。用长焦距的透镜聚焦二次谐波，得到34mW，208nm的紫外脉冲激光。从实验上详细研究了基频光的线宽对二次谐波效率的影响，二次谐波的线宽对四次谐波效率的影响、基频光的波长对四次谐波产生效率的影响。全面地分析了单片和组合大拆射滤光片及抽运功率对钛蓝宝石激光器基频光的脉宽、线宽等参量的影响。     

Accuracy comparison of absolute optical frequency measurement between harmonic-generation synthesis and a frequency-division femtosecond comb

Using an iodine-stabilized He-Ne laser as a transfer oscillator, we compare absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb. The agreement between these two measurements, both linked to the Cs standard, is 220+/-770 Hz, yielding a measurement accuracy of 1.6x10(-12). We report 473 612 353 604.8+/-1.2 kHz as a preliminary updated value of the absolute frequency of the " f" component for the He-Ne laser international standard at 633 nm.     

高能量高转换效率355 nm紫外激光器

为了得到一种三倍频效率高达60%的355 nm脉冲激光器,采用曲率半径分别为2 m的凹凸高斯镜和9 m的平凹全反镜组合作为谐振腔,加以电光调Q,得到1 064 nm高光束质量激光输出,再将其进行行波放大,获得重复频率10 Hz、脉宽7.3 ns、单脉冲能量1.01 J的1 064 nm基频光输出。利用Ⅰ类相位匹配LBO晶体进行二倍频、Ⅱ类相位匹配LBO晶体进行三倍频以得到波长为355 nm的紫外光输出。通过二倍频和三倍频输出特性和非线性晶体参数的分析和实验调试,最终获得了单脉冲能量为608 mJ、脉宽为5.7 ns、线宽为2 nm的紫外激光输出。通过优化二倍频的转换效率,可使1 064 nm基频光到三倍频得到的355 nm紫外光的转换效率达60%。     

光梳主动滤波放大实现锶原子光钟二级冷却光源

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.     

利用椭圆高斯光束产生266nm紫外连续激光

本文采用商用532 nm激光器作为基频光源,利用偏硼酸钡(β-BBO)晶体进行外腔倍频,实现了266 nm连续激光的高效输出.文中详细模拟了BBO晶体中的束腰形状对倍频效率的影响,仿真和实验结果均表明椭圆高斯光束可以有效改善走离效应,提高倍频转换效率.通过优化蝶形倍频腔,可以使椭圆高斯光束在腔内共振,当1 W基频光输入时可输出约180 mW的266 nm紫外连续激光,倍频转换效率达到18%.     

Stable double spacing multiwavelength Brillouin-Erbium doped fiber laser based on highly nonlinear fiber

A double frequency spaced multiwavelength Brillouin-Erbium doped fiber laser (BEDFL) with figure-of-eight cavity have been successfully developed and tested. Double frequency spacing is achieved by using a piece of 2 km of highly nonlinear fiber (HNLF) as a gain medium. Figure-of-eight configuration removes the odd order Stokes signals via a four-port circulator. Fifteen Stokes channels are simultaneously generated with a spacing of 0.154 nm that is around 20 GHz, when the Brillouin pump and 980 nm pump powers are fixed at the optimized values of 6 dBm and 40 mW, respectively. Fourteen anti stoke channels are also obtained, which are generated through four wave mixing (FWM) process in the laser cavity. The output is smooth triangular comb. The BEFL can also be tuned from 1526.5 to 1567.5 nm.     

Frequency comb generation using femtosecond pulses and cross-phase modulation in optical fiber at arbitrary center frequencies

A technique is presented for generating optical frequency combs centered at arbitrary wavelengths by use of cross-phase modulation (XPM) between a femtosecond pulse train and a cw laser beam by copropagating these signals through an optical fiber. We report results from use of this method to place a 90-MHz frequency comb on an iodine-stabilized Nd:YAG laser at 1064 nm and on a frequency-doubled Nd:YVO(4) laser at 532 nm. XPM is verified to be the comb-generating process, and the width of the frequency comb is measured and compared with theory. The spacing of the frequency comb is compared with the femtosecond source, and a frequency measurement with this comb is demonstrated.     

Frequency comb generation in the green using silicon nitride microresonators

Optical frequency combs enable precision measurements in fundamental physics and have been applied to a growing number of applications, such as molecular spectroscopy, LIDAR and atmospheric trace‐gas sensing. In recent years, the generation of frequency combs has been demonstrated in integrated microresonators. Extending their spectral range to the visible is generally hindered by strong normal material dispersion and scattering losses. In this paper, we report the first realization of a green‐light frequency comb in integrated high‐Q silicon nitride (SiN) ring microresonators. Third‐order optical non‐linearities are utilized to convert a near‐infrared Kerr frequency comb to a broadband green light comb. The 1‐THz frequency spacing infrared comb covers up to 2/3 of an octave, from 144 to 226 THz (or 1327‐2082 nm), and the simultaneously generated green‐light comb is centered around 570‐580 THz (or 517‐526 nm), with comb lines emitted down to 517 THz (or 580 nm) and up to 597 THz (or 502 nm). The green comb power is estimated to be as high as −9.1 dBm in the bus waveguide, with an on‐chip conversion efficiency of −34 dB. The proposed approach substantiates the feasibility of on‐chip optical frequency comb generation expanding to the green spectral region or even shorter wavelengths.

     

Cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultrahigh detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The long cavity lifetime dramatically enhances the effective interaction between the light field and intracavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. The use of low-dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of ～?10% of the actual optical frequency. The light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from several gigahertz to hundreds of kilohertz. In this review we will discuss the principle of cavity-enhanced direct frequency comb spectroscopy and the various implementations of such systems. In particular, we discuss several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. We present several cavity-comb coupling methods to take advantage of the broad spectral bandwidth and narrow spectral components of a frequency comb. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams. These results demonstrate clearly that the wide bandwidth and ultrasensitive nature of the femtosecond enhancement cavity enables powerful real-time detection and identification of many molecular species in a massively parallel fashion.