Compact frequency-comb Fourier-transform infrared spectrometer

We demonstrate a Fourier-transform infrared (FTIR) spectrometer without moving parts based on frequency-combs where the interference of two coherent frequency-comb beams occurs via wavefront combination, thereby eliminating the need of a dielectric combiner. This enables dispersionless operation over eventually the complete THz-to-visible spectrum in a single instrument. Furthermore we demonstrate the use of a single GaSe crystal for generating both beams, in an implementation for the 22-36 THz mid-infrared range. Spurious cross-modulation effects and a way to eliminate them are experimentally demonstrated. The compact comb-FTIR can be equally well applied for spectroscopic microscopy as for long-path standoff chemical monitoring.     

A mid-infrared spectrometer based on difference frequency generation

A mid-infrared spectrometer operating in the range from 3.42 to based on difference-frequency mixing of a tunable Ti:Sapphire laser and a diode-pumped monolithic Nd:YAG laser in periodically poled LiNbO₃ (PPLN) crystals was developed. A system line width of about 36 MHz was obtained. An energy conversion efficiency of 0.6 mW/W² was obtained at . The direct absorption spectra of pure methane and pure carbon dioxide samples were measured to test the performance of the spectrometer.     

Time-domain interferometry for direct electric field reconstruction of mid-infrared femtosecond pulses

Mid-infrared ultrashort pulses of 9.2-microm center wavelength are characterized in both amplitude and phase. This is achieved by use of a variant of spectral phase interferometry for direct electric field reconstruction in which spectral interferometry has been replaced with time-domain interferometry, a technique that is well suited for infrared pulses. The setup permits simultaneous recording of the second-order interferometric autocorrelation, thus providing an independent check on the retrieved spectral phase.     

Time-domain surface profile imaging via a hyperspectral Fourier transform spectrometer

We describe a method for time-domain surface profile measurements via white-light reflection spectroscopy using a hyperspectral Fourier transform spectrometer (HS-FTS). This technique measures the frequency of the spectral modulation of reflected light from a multilayer optical surface and reports the spatially resolved optical thickness. Owing to the Fourier relationship, the Fourier transform spectrometer manifests this spectral modulation as temporal satellites in interferogram space. We show that measurement of the positions of these satellites can be used to reconstruct the optical thickness profile over a surface using the HS-FTS.     

Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer

We demonstrate the possibility of extending the well-established metrological performance of optical frequency-comb synthesizers to the mid-IR region by phase locking the pump and signal lasers of a difference-frequency source to two near-IR teeth of an optical comb. An uncertainty of 800 Hz (1.1 x 10(-11)) in the absolute frequencies of CO2 transitions near 4.2 microm has been measured by cavity-enhanced saturated-absorption spectroscopy. Prospects for the creation of a new dense set of high-quality molecular frequency standards in the IR are discussed.     

A DFG-based cavity ring-down spectrometer for trace gas sensing in the mid-infrared

Continuing studies into an all-diode laser-based 3.3 μm difference frequency generation cavity ring-down spectroscopy system are presented. Light from a 1,560 nm diode laser, amplified by an erbium-doped fibre amplifier, was mixed with 1,064 nm diode laser radiation in a bulk periodically poled lithium niobate crystal to generate 16 μW of mid-IR light at 3,346 nm with a conversion efficiency of $0.05\,\%\,{\text{W}}^{-1}\,{\text{cm}}^{-1}$ . This radiation was coupled into a 77 cm long linear cavity with average mirror reflectivities of 0.9996, and a measured baseline ring-down time of $6.07\pm 0.03\,\upmu{\rm s}$ . The potential of such a spectrometer was illustrated by investigating the $P(3)$ transition in the fundamental $\nu_{3}(F_{2})$ band of ${\text{CH}}_4$ both in a 7.5 ppmv calibrated mixture of ${\text{CH}}_4$ in air and in breath samples from methane and non-methane producers under conditions where the minimum detectable absorption coefficient ( $\alpha_{\rm min}$ ) was $2.8 \times 10^{-8}\,{\rm cm}^{-1}$ over 6 s using a ring-down time acquisition rate of 20 Hz. Allan variance measurements indicated an optimum $\alpha_{\rm min}$ of $2.9\times 10^{-9}\,{\rm cm}^{-1}$ over 44 s.     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb Hz^-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com     

A broadband mid-infrared laser magnetic resonance spectrometer for the spectral range of 1200–2000 wavenumbers

A broadband mid-infrared laser magnetic resonance spectrometer is described, which can be operated in intracavity or extracavity configuration. The specially designed CO-laser gain tube allows operation from 1220–2000 cm^?1. Experimental details and examples of spectroscopic applications are discussed.     

Phase-locked white-light continuum pulses: toward a universal optical frequency-comb synthesizer

We demonstrate that two white-light continuum pulses that are independently generated by phase-locked ultrashort laser pulses are locked in phase and show surprisingly clear and stable Young interference fringes. The experiment shows that the two generated continua emit essentially in phase and that random phase jitter can remain negligible. This result is not only of interest for studies of nonlinear field-matter interactions but also suggests that such white-light continuum pulses can be used to realize a broad frequency comb for absolute frequency measurements from the IR to the UV.     

Multi-level effects in the high-order harmonic generation driven by intense frequency-comb laser fields

High harmonic generation(HHG) driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase(CEP) shift, without increasing the driving intensity. However,the multiphoton-resonant enhancement(MRE) factor in the realistic atomic hydrogen is much smaller than that in a twolevel system. To study the deviation, we present a theoretical investigation of the multiphoton resonance dynamics of three-level systems driven by intense frequency-comb laser fields. The many-mode Floquet theorem(MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between the quantum system and the laser fields. The investigations show that the dipole interaction of a two-level system with the third level affects the multiphoton resonance dynamics and enhances the HHG spectra. It is the dipole interaction of the excited level of the two-level system with other levels that results in the smaller MRE factor in the realistic atomic system.     

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.     

Direct frequency-comb spectroscopy of a dipole-forbidden clock transition in trapped 40Ca+ ions

We demonstrate direct frequency-comb (FC) spectroscopy of the dipole-forbidden 4s(2)S(1/2)-3d(2)D(5/2) transition in trapped (40)Ca(+) ions using an unamplified FC laser. The excitation is detected with nearly 100% efficiency using a shelving scheme in combination with single-ion imaging. The method demonstrated here has the potential to reach hertz-level accuracy, if a hertz-level linewidth FC is used in combination with confinement in the Lamb-Dicke regime.     

多光束泵浦中红外激光器

为了获得高功率高效率3 m～5 m中红外激光输出,利用双声光调Q晶体,通过高重复频率驱动调Q同步技术和LD侧面泵浦双棒串接技术,获得高功率高光束质量1.06 m激光双端输出,外置起偏器获得4束激光输出,利用波片偏振旋光原理,实现4束偏振态一致的激光输出,泵浦非线性晶体PPLT进行频率变换,实现高功率3 m～5 m中红外激光输出。在电源输入电流30 A、调Q驱动频率10 kHz的条件下,获得最高功率10.6 W的3.9 m中红外激光,1.06 m～3.9 m转化效率为9.5%。实验结果表明:通过双声光调Q技术和LD侧面泵浦双棒串接技术,可以实现4束高重复频率窄脉宽1.06 m偏振激光输出,泵浦PPLT可获得高功率3.9 m中红外激光输出。     

Time-domain sweeplets for acoustic measurements

This paper proposes new types of time-domain generated sine sweeps for impulse response measurements. A general time-domain analytical formulation method, combined with numeric phase alignment and frequency-domain inverse filtering is presented. It is applied to derive three new families of controllable spectrum sine sweeps called sweeplets, capable of matching 1/f^β background noises, producing finite band defocusing and single frequency focusing shapes. Mathematical properties concentrating on practical control of the signal shapes are examined. Effects of various perturbations, such as stationary and transient background noise, harmonic distortion and finite length are presented. Applicability of the proposed method is experimentally verified by a room acoustic measurement example.     

Time-domain nature of group delay

The characteristic of group delay is analyzed based on an electronic circuit, and its time-domain nature is studied with time-domain simulation and experiment. The time-domain simulations and experimental results show that group delay is the delay of the energy center of the amplitude-modulated pulse, rather than the propagation delay of the electromagnetic field. As group velocity originates from the definition of group delay and group delay is different from the propagation delay, the superluminality or negativity of group velocity does not mean the superluminal or negative propagation of the electromagnetic field.     

Time-domain differentiation of terahertz pulses

We report on the time-domain differentiation of light waves by metallic transmission gratings. Time-resolved terahertz experiments show that the first time derivative of an arbitrary waveform can be achieved by use of gratings of subwavelength period. The results are in accord with classical diffraction theory and may permit novel applications for tailoring few-cycle light pulses and ultrahigh-frequency optoelectronics.     

Time-domain fiber laser hydrogen sensor

We report a novel scheme for a fiber-optic hydrogen sensor based on an erbium-doped fiber laser with a palladium-coated tapered fiber within the laser cavity. The tapered fiber acts as a hydrogen-sensing element. When the sensing element is exposed to a hydrogen atmosphere, its attenuation decreases, changing the cavity losses and leading to a modification of the laser transient. The hydrogen concentration is obtained by simple measurement of the buildup time of the laser. This technique translates the measurement of hydrogen concentration into the time domain, and it can be extended to many intensity-based fiber sensors. Relative variations in the buildup time of up to 55% at an increase of the hydrogen concentration from 0 to 10% are achieved with a resolution of better than 0.1%.     

Time-domain programmable digital multi-beamforming system

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中红外固体激光器研究进展

本文介绍了中红外固体激光器中新型的Fe²⁺: ZnSe激光器和基于MgO: PPLN、ZnGeP₂晶体的光参量振荡器的发展现状,讨论了它们在发展过程中遇到的技术难题,探讨了中红外固体激光器的未来发展方向。波长为3~5 μm的中红外固体激光器具有效率高、体积小和重量轻等优点,在工业、医疗、军事等方面具有重要应用价值,研制大尺寸、高质量中红外激光晶体和输出波长更长的红外高功率激光泵浦源已成为中红外固体激光器未来发展方向之一。     

Time-domain Fresnel-to-Fraunhofer diffraction with photon echoes

A photon echo experiment in Tm(3+):YAG is reported that shows, for the first time to the authors' knowledge, the time-domain equivalent of the transition from near- to far-field diffraction, including Talbot self-imaging effects. The experiment demonstrates the huge dispersion capability of photon echoes and opens the way to further exploration of space-time duality.