腔内带圆形空心波导的可调谐窄带脉冲染料激光器

本文报道了一种腔内带圆形空心波导和掠入射光栅的可调谐窄带脉冲染料激光器;实验结果表明:该激光系统能获得基横模、窄频带和高效率的激光输出。文中对系统设计机理也进行了分析。     

A tunable,single mode,injection-locked flashlamp pumped dye laser

A tunable, very narrow bandwidth, flashpumped dye laser is described. Publse energies up to 50 mJ at a bandwidth of less than 30 MHz, and with a divergence of 0.4 mrad, are obtained. Spectral narrowing and tuning is achieved by injection locking of the pulsed laser to the radiation, of a tunable cw dye laser. The cavity is designed for single longitudinal and transverse mode operation, with a mode diameter large enough to fill the active medium. This work was supported in part by the Israel commission for Basic Research, and in part by a grant from the United States Israel Binational Science Foundation (BSF), Jerusalem, Israel.     

Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity

The single mode pulsed dye laser is an attractive tool for many spectroscopic applications. Long cavity tunable dye lasers generally operate in multi-longitudinal modes within the bandwidth of gain profile. Single longitudinal mode oscillation can be obtained by either making the cavity short enough or introducing an additional loss mechanism, in which all modes but one have a gain less than their loss. A new technique to achieve single mode operation in a long cavity dye laser, based on Rhodamine 6G dye in ethanol and ethylene glycol solution, pumped by a high repetition rate copper vapor laser, is reported. This laser, which operates in three modes in grazing incidence grating configuration (cavity length of 16 cm), has been made to lase in single mode by increasing the loss in the resonator through beam walk-off.     

Axial mode tuning of a single frequency Yb:YAG thin disk laser

We describe an arrangement for an Yb:YAG thin disk laser, which enables narrow bandwidth operation in single-frequency mode at freely selectable wavelengths within the broad tuning range of the laser. This is facilitated by a combination of a double-stage birefringent filter and an etalon inside the laser cavity. We investigate the wavelength selection characteristics of the single elements as well as their combination. A simple procedure is implemented for a computer-based automation of wavelength tuning. The reflectivity of the partially reflecting resonator mirror is optimised, and the laser pump power is adapted for best tuning performance. Single-frequency emission is achieved in a frequency range of 9.75 THz (wavelength range 1020 nm to 1055 nm). Each axial laser mode in this range can be selected individually. The axial mode separation of 0.47 GHz corresponds to wavelength steps of 1.7 pm at 1030 nm. PACS 42.55.Xi; 42.60.Fc; 42.60.Lh     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C₂H₂) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS. Received: 22 October 2001 / Revised version: 16 January 2002 / Published online: 14 March 2002     

双重频率锁定的腔衰荡吸收光谱技术及信号处理

针对传统腔衰荡光谱技术浓度获取率低,提出基于双重锁定的连续波腔衰荡吸收光谱技术.通过波长调制一次谐波信号将激光器的频率锁定到C_2H_2吸收线上,同时使用PDH锁频技术将衰荡腔锁定到激光器上,从而避免了测量过程中激光器的频率漂移和腔长的抖动,使测量结果更加精确;并且,由于双重锁定,单次衰荡事件的发生率,也就是浓度信息的获取率只受衰荡时间以及重新锁定时间限制,在本试验系统中采集速率可以达到30 k Hz,可以实现对气体浓度的快速测量.为了提高信噪比,采用Kalman滤波技术,对浓度信息进行实时处理,有效抑制了噪声,根据阿伦方差分析,探测灵敏度可以达到4×10~(-9)cm~(-1)(2 s平均).     

Injection seeded single mode intra-cavity absorption spectroscopy

We present a single mode intra-cavity spectroscopy system in which the test laser is locked to a narrow band external single mode laser. This technique solves many problems typically encountered in single mode intra-cavity spectroscopy: it results in good tuning properties, a stable single mode operation close to the lasing threshold, a high side-mode suppression and a reduction of spontaneous emission without the use of any frequency selective element. Measurements of broadband absorptions as well as measurements of a narrow band absorption line of the oxygen A-band are presented and compared with theoretical model predictions. The prototype described in this work provides an enhancement in sensitivity of approximately a factor of 12, and it demonstrates the influence of optical injection to single mode intra-cavity spectroscopy. As there is no need for any frequency selective element inside the cavity, the sensitivity can be massively enhanced by optimizing the laser cavity.     

Continuously tunable, high-power, single-mode radiation from a short-pulse free-electron laser

This paper gives the first demonstration of high-power, continuously tunable, narrowband radiation that is produced by means of a free-electron laser (FEL) in the far-infrared region of the electromagnetic spectrum. A Fox-Smith intracavity étalon was used to induce phase coherence between the 40 optical micropulses that were circulating in the laser cavity. The corresponding phase-locked spectrum consisted of a comb of discrete frequencies separated by 1 GHz. A pair of external Fabry-Pérot étalons was used to filter out a single line from this spectrum. The power in the selected narrow line at 69 microm wavelength was equal to 250 mW during the macropulse of the laser. The spectral width of the selected line is as small as that of a single cavity mode, i.e., a fraction of 25 MHz, in single macropulses of the laser. The average bandwidth of 25 MHz is determined by mode hopping of the phase-locked FEL. The selected frequency hops over 25 MHz between the extrema of this band. The influence of partially coherent spontaneous emission and mode hopping on the final linewidth was studied. The narrow-linewidth radiation was scanned in frequency over 1 GHz. We show that the possibilities to scan over smaller or larger frequency intervals are unlimited.     

Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser

A novel cavity ringdown spectrometer, incorporating a miniature continuous-wave swept-frequency laser that is widely tunable, requires less than 1 s to record wide-ranging absorption spectra with high sensitivity in a single rapid sweep of the laser frequency. The free spectral range of the ringdown cavity defines a sampling grid to measure absorbance-dependent ringdown times at successive cavity-resonance frequencies. The spectrometer has a single-ended transmitter-receiver configuration based on retro-reflected optical-heterodyne detection and exploiting fibre-optical telecommunications components. This swept-frequency approach to cavity ringdown spectroscopy yields a simple, low-cost, compact, rugged, versatile instrument for efficient sensing of gases. The performance of the spectrometer is demonstrated by measuring weak near-infrared rovibrational spectra of carbon dioxide gas within the 1.5–1.6 m wavelength range. PACS 42.62.Fi; 07.57.Ty; 07.07.Df     

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.     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Flashlamp pumped tunable dye laser of ultra-narrow bandwidth

A flash lamp pumped dye laser with a ring cavity is injected with the light beam coming from a stabilized singlemode c.w. dye laser. The light emitted by the pulsed laser is analysed either by a Fabry Perot etalon, or by producing a Doppler-free two-photon transition. The bandwidth of the pulsed laser is identical to the one of the c.w. laser; but the pulsed light is shifted to higher frequency by an amount of a few MHz.     

Absorption spectroscopy: technique provides extremely high sensitivity

Technology associated with cavity ringdown laser absorption spectroscopy is reviewed. The technique is used to study general trace analysis, free radicals in flames and chemical reactors, molecular ions in electrical discharges, biological molecules and water clusters in supersonic jets, and vibrational overtones of stable molecules. Its specific enough to detect about 1-ppm fractional absorption by a gaseous sample in about 10 microseconds. The use of mirrors in ringdown sepctroscopy is explained. Other topics include the generation of pulsed infrared rays and the adaptation of ringdown spectroscopy for use with narrow-bandwidth continuous-wave lasers.     

Theoretical investigation on the intracavity Doppler effect in continuous wave swept-cavity ringdown spectroscopy

We theoretically investigate the spectral measurement errors that are apt to occur in continuous wave (CW) cavity ringdown (CRD) techniques suffering intracavity Doppler shifts. In the typical CW CRD scheme based on a cavity sweep operation for the resonant coupling of a probe laser into a ringdown cavity, the intracavity probe light detunes gradually over time, carrying time-dependent loss information of an absorption feature. Frequency-drifting ringdown signals are theoretically modelled and found to result in erroneous absorption spectra that exhibit the frequency shift of absorption profiles as well as linewidth broadening. PACS 42.62.Fi; 42.60.Da; 42.25.BS     

Precision continuous-wave cavity ringdown spectroscopy of CO<Subscript>2</Subscript> at 1064 nm

A high performance continuous-wave cavity ringdown spectrometer using a rapidly swept cavity has been applied to investigate weak absorption of CO₂ line near 1064 nm. In the experimental setup, offset locking of the Nd:YAG laser to other iodine-stabilized Nd:YAG laser provides a frequency accuracy better than 20 kHz. The ringdown time was obtained by fitting the experimental ringdown data to a theoretically obtained ringdown curve. The absorption coefficient and absolute center wavelength are determined with accuracy better than two orders of magnitude with respect to the previous observations. Our experimental setup yields high performance in a relatively simple, low cost, and compact system that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.     

Mode-by-mode optical feedback: cavity ringdown spectroscopy

We describe a new continuous wave cavity ringdown spectroscopy (cw-CRDS) approach using an extended cavity diode laser (ECDL) optically self-locked to a high finesse cavity including an intracavity glass plate under the Brewster angle. Low noise, mode-by-mode absorption spectra are recorded at a high acquisition rate (laser frequency scan greater than 400 GHz/s) and covering four orders of magnitude in absorption coefficient. Sampling spectra with the fixed high finesse cavity frequency comb provides high precision frequency markers. An original scheme for the laser beam shut-down, based on signal shape analysis and the diode laser injection current control, is presented. This scheme avoids any supplementary switching device. To retrieve ringdown processing at a kilohertz rate several exponential decay fit algorithms are compared. Performances of this new scheme are demonstrated with the observation of very weak lines of the oxygen B-band around 680 nm. Atmospheric spectra of isolated lines averaged for less than 10 s show a baseline noise of 5×10^-10 cm^-1 and a single point minimum detectable absorption loss over a one-second measurement interval of 2×10^-10 cm/ is obtained. PACS 07.88.+y; 42.55.Px; 42.62.Fi     

Theoretical analysis on the dynamic absorption saturation in pulsed cavity ringdown spectroscopy

We perform a theoretical analysis on the transient dynamics of absorption saturation in pulsed cavity ringdown spectroscopy. Based on a coupled rate equation approach, modelling of the dynamic saturation is carried out to account for time-evolving intracavity photon density and absorbing population. Master equations are derived in terms of normalized system parameters, which enables one to systematically study the non-exponential feature of saturated cavity ringdown signals. Erroneous quantification of sample absorbance by the conventional ringdown time analysis is numerically simulated, and the saturated spectral feature showing a Lamb dip is obtained for a Doppler broadened sample. Finally a novel numerical recipe is proposed to handle saturated ringdown signals and retrieve unsaturated spectra, allowing relevant absorption parameters without degrading the measurement signal-to-noise ratio. PACS PACS 42.62.Fi; 42.60.Da     

用于光频传递的通信波段窄线宽激光器研制及应用

通信波段窄线宽激光器在基于光纤的光学频率传递中有着重要应用. 本文报道了1550 nm超窄线宽光纤激光器的研制及其在光学频率传递中的初步应用结果. 利用一台激光光源, 分别锁定到两个参考腔上(精细度分别为344000和296000), 锁定后经拍频比对测得单台激光线宽优于1.9 Hz, 秒级频率稳定度为1.7×10^-14, 优于国内同类报道. 将研制的超窄线宽激光器用于光纤光学频率传递, 在50 km光纤盘上实现了 7.5×10^-17/s的传递稳定度, 较采用商用光纤激光器提高了3.2倍.     

一种双反射壁型二维光子晶体窄带滤波器

提出了一种基于双反射壁型结构的光子晶体窄带滤波器, 该滤波器由一个单模谐振腔和两个反射壁构成. 通过调节谐振腔与反射壁之间的距离可以改变滤波谱线的带宽. 利用时域有限差分法进行仿真分析, 结果表明, 该滤波器的工作频率在193.40 THz附近, 带宽小于5.9 GHz, 峰值透射率高达94%, 工作区域长度仅有9 μm. 该器件可应用于密集波分复用系统.