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Sub-Doppler resolution limited Lamb-dip spectroscopy of NO with a quantum cascade distributed feedback laser
Optics Express
Abstract
A quantum cascade distributed feedback laser operating at 5.2 microm is used to obtain sub-Doppler resolution limited saturation features in a Lamb-dip experiment on the R(13.5)1/2 and R(13.5)3/2 transitions of NO. The dips appear as transmission spikes with full widths of ~ 4.3 MHz. At this resolution the 73 MHz _-doubling of the R(13.5)3/2 line, which is normally obscured by the 130 MHz Doppler broadening, is easily resolved.
... Saturation spectroscopy encompasses a variety of laser based techniques most commonly aimed at removing Doppler broadening and probing molecular hyperfine structures of low pressure gas samples. [1][2][3] These include Lamb-dip and polarisation spectroscopies, which also find applications in laser frequency stabilization, 4-7 linewidth measurements, 8,9 in plasma diagnostics for determining electric field strengths from Stark splitting, 10 and in the measurement of transition dipole moments. 3 While saturation often relies upon intense (saturating) radiation, which was initially provided by high power lasers, it can also be achieved with lower power continuous wave (cw) diode lasers by probing atomic transitions whose absorption cross sections are several orders of magnitude larger than molecular cross sections. ...
We report on the observation of saturation effects in Intracavity Faraday Modulation Spectroscopy (INFAMOS). A quantum cascade laser operating at ∼5.3 µm is used to probe the 2 Π 3/2 and 2 Π 1/2 R(3.5) transitions in the fundamental band of nitric oxide. With average intracavity intensities up to 450 W cm −2 , the saturation of these molecular transitions is observed up to a total pressure of ∼240 Torr. The experimental data are interpreted by incorporating saturation into a model for the INFAMOS line shape in the homogeneously broadened limit. Published by AIP Publishing.
... This is due to their high optical emission power, narrow linewidths, and near roomtemperature operation [4]. High-resolution spectroscopic techniques such as saturation spectroscopy benefit from these narrow linewidths, and are tools for establishing accurate frequency markers and laser stabilization [5,6]. The high coherence and intensity of the electric field produced by these sources also allows the observation of coherent transient effects when the laser frequency is swept over a strong molecular transition of a velocity selected sample on a timescale that is short compared to its relaxation time [7][8][9]. ...
We report on the broadening of the optical bandwidth of a distributed feedback quantum cascade laser (QCL) caused by the application of radio frequency (RF) noise to the injection current. The broadening is quantified both via Lamb-dip spectroscopy and the frequency noise power spectral density (PSD). The linewidth of the unperturbed QCL (emitting at ∼ 5.3 μm ) determined by Lamb-dip spectroscopy is 680 ± 170 kHz , and is in reasonable agreement with the linewidth of 460 ± 40 kHz estimated by integrating the PSD measured under the same laser operating conditions. Measurements with both techniques reveal that by mixing the driving current with broadband RF noise the laser lineshape was reproducibly broadened up to ca 6 MHz with an increasing Gaussian contribution. The effects of linewidth broadening are then demonstrated in the two-color coherent transient spectra of nitric oxide.
... Sub-Doppler resolution molecular spectroscopy in the mid-infrared region has made remarkable progress due to the development of tunable and intense sources with a narrow spectral width (e.g., difference-frequency-generation (DFG) [1][2][3][4][5][6][7], quantum cascade lasers [8][9][10], and optical parametric oscillators (OPOs) [11][12][13][14]). Some spectrometers have been combined with an optical frequency comb (OFC) to measure and/or control the source frequency with an uncertainty corresponding to a fraction of the spectral width of the Lamb dips. ...
We present a novel scheme of frequency scan and wavelength modulation of a difference-frequency-generation source for comb-referenced sensitive spectroscopy. While the pump and signal frequencies are phase-locked to an optical frequency comb (OFC), the offset frequency between the signal wave and the nearest comb tooth is modulated to apply a wavelength-modulation technique, and the idler wave frequency is repeatedly swept for signal accumulation by changing the repetition frequency of the OFC. The spectrometer is applied to absolute frequency measurement of weak hyperfine-resolved rovibration transitions of the ν1 band of CH3I, and the uncertainty in frequency determination is reduced by one order of magnitude in compared with that of the previous work published in Optics Express 20, 9178-9186 (2012).
... Microwave sideband CO 2 -and CO-laser systems [18,19] contributed to extending the tunable range in the 10 and 5 μm regions, but no complete analysis of a vibrational band has been reported thus far. Recent progress of mid-infrared sources such as quantum cascade laser diodes [20], optical parametric oscillators [21], and difference-frequencygeneration (DFG) in waveguide devices of nonlinear optical elements [22] has extended the frequency range to which sub-Doppler resolution spectroscopy can be applied. It reduces the observed spectral linewidth to a level similar to that of microwave spectroscopy. ...
We determine the absolute frequencies of 56 rotation-vibration transitions of the ν(3) band of CH(4) from 88.2 to 90.5 THz with a typical uncertainty of 2 kHz corresponding to a relative uncertainty of 2.2 × 10(-11) over an average time of a few hundred seconds. Saturated absorption lines are observed using a difference-frequency-generation source and a cavity-enhanced absorption cell, and the transition frequencies are measured with a fiber-laser-based optical frequency comb referenced to a rubidium atomic clock linked to the international atomic time. The determined value of the P(7) F(2)((2)) line is consistent with the International Committee for Weights and Measures recommendation within the uncertainty.
... Moreover, QCLs have been assembled in external-cavity configurations[14]and have been frequency stabilized by electronic servo-locking[15,16,17]and phase-locking techniques[18]. Applications of QCLs for both sensitive trace-gas detection[19,20]and high-resolution subDoppler spectroscopy[21,22]have been recently demonstrated. In the last years, the high versatility of these sources has allowed us to perform both sensitive detection on N 2 O and CH 4 by frequency-modulation spectroscopy[23]and Doppler-limited comb-assisted absolute frequency measurements of CO 2 transitions[24]. ...
The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 microm has been long-term stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal. Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz.
... An alternative approach for facilitating a better spatial resolution might be in-situ pump probe experiments. Although saturated absorption spectroscopy has so far mainly been applied in the field of frequency metrology [91,92] as a Doppler-free method, it could be used to obtain localized concentration information from plasmas. It has been shown [93] and partly discussed in this paper that the output power levels of QCLs are sufficiently high to obtain power saturation. ...
The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.
... The drawbacks of these laser sources (cryogenic temperatures, low spectral purity, etc.) have stimulated the recent development of new tunable coherent sources, such as Optical Parametric Oscillators (OPO) [1], Difference-Frequency Generators (DFG) [2], and Quantum Cascade Diode Lasers (QCDL) [3]. All these sources have been extensively used in many high sensitivity detection studies of molecular species [4][5] and in few Doppler-free investigations of molecular lines [6][7][8]. In this work we report, according to our knowledge, the first Doppler-free spectroscopy of an atomic species (xenon atom) by using low-power difference-frequency radiation. ...
We report on the first Doppler-free spectroscopy investigation of an atomic species, xenon, performed in the mid-infrared using difference-frequency radiation. The absorption saturated spectrum of the xenon 6p[3/2]2?5d[5/2]3 transition (2p6?3d'1 in Paschen notation) at 3.1076 microm was investigated using about 60 microwatts of cw narrowband radiation (Deltanu=50 kHz) generated by difference-frequency mixing in a periodically-poled Lithium Niobate crystal. A single frequency Ti:Sapphire laser (power 800 mW) and a monolithic diode-pumped Nd:YAG laser (300 mW) were used as pump and signal waves respectively. We used natural enriched xenon, which contains nine stable isotopes, two of which, 129Xe and 131Xe, exhibit a hyperfine structure owing to their nuclear spin. The small isotope displacements expected for this atom and the complex hyperfine structure of the odd isotopes make it difficult to fully resolve the recorded saturated-absorption spectra. In spite of this, we have been able to analyze the isolated 129Xe F''=5/2?F'=7/2 hyperfine component by means of first-derivative FM spectroscopy.
... Quantum cascade lasers 1,2 (QCLs) emitting in the mid-to long-wave infrared have greatly increased the availability of tunable lasers in this spectral region. Applications for such laser sources include highresolution spectroscopy, 3,4 optical communications, 5 and chemical sensing. 6 In this Letter we extend previous research on linewidth narrowing and frequency stabilization of QCLs, both free running 7 and locked to a molecular transition, 8 by locking these lasers to an optical cavity with the Pound -Drever-Hall technique. ...
We report a heterodyne beat with a linewidth of 5.6+/-0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 microm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop.
... Remillard et al. have used a QC-DFB laser operating at 5.2 m to obtain sub-Doppler resolution-limited saturation features in a Lamb-dip experiment on the R(13.5) and R (13.5) transitions of NO [41]. The laser is operated CW in a LN Dewar. ...
Recent advances and new directions in quantum cascade (QC) lasers are discussed. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among midinfrared semiconductor lasers in terms of wavelength agility as well as power and temperature performance. Because of the cascaded structure, QC lasers have a slope efficiency proportional to the number of stages. Devices with 100 stages having a record peak power of 0.6 W at room temperature are reported. QC lasers in the AlInAs-GaInAs lattice matched to InP material system can now be designed to emit in the whole midinfrared range from 4 to 20 μm by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 μm have been produced. New results on QC lasers emitting at 19 μm, the longest ever realized in a III-V semiconductor laser, are reported. These devices use innovative plasmon waveguides to greatly enhance the mode confinement factor, thereby reducing the thickness of the epitaxial material. By use of a distributed feedback (DFB) geometry, QC lasers show single-mode emission with a 30-dB side-mode suppression ratio. Broad continuous single-mode tuning by either temperature or current has been demonstrated in these DFB QC lasers at wavelengths in two atmospheric windows (3-5 and 8-13 μm), with continuous-wave linewidths <1 MHz when free running and ∼10 KHz with suitable locking to the side of a molecular transition. These devices have been used in a number of chemical sensing and spectroscopic applications, demonstrating the capability of detecting parts per billion in volume of several trace gases. Sophisticated band-structure engineering has allowed the design and demonstration of bidirectional lasers. These devices emit different wavelengths for oppos- ite bias polarities. The last section of the paper deals with the high-speed operation of QC lasers. Gain switching with pulse widths ∼50 ps and active modelocking with a few picosecond-long pulses have been demonstrated. Finally, a new type of passive modelocking has been demonstrated in QC lasers, which relies on the giant and ultrafast optical Kerr effect of intersubband transitions.
... High power and the potentially narrow spectral linewidth of QC lasers open up the possibility to use these devices in nonlinear laser spectroscopy. The first experimental demonstration of nonlinear light-matter interaction involving a QC-DFB laser 2 as a light source was reported in [44]. In this paper it was demonstrated that a free-running QC-DFB laser operated in a rapid scan mode has sufficient beam quality, power, and linewidth to produce sub-Doppler saturation features in a simple Lamb-dip geometry. ...
There is an increasing need in many chemical sensing applications
ranging from industrial process control to environmental science and
medical diagnostics for fast, sensitive, and selective gas detection
based on laser spectroscopy. The recent availability of novel pulsed and
CW quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared
spectroscopic sources address this need. A number of spectroscopic
techniques have been demonstrated worldwide by several groups. For
example, the authors have employed QC-DFB lasers for the monitoring and
quantification of several trace gases and isotopic species in ambient
air at ppmv and ppbv levels by means of direct absorption, wavelength
modulation, and cavity enhanced and cavity ringdown spectroscopy
We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.
The linewidth of an external cavity quantum cascade laser is studied as a function of injection current and laser scan rate. The laser linewidth is inferred to be ca. 2.5 MHz from Lamb-dip spectra on a low pressure sample of NO and its variation with injection current is well modeled using literature values for the intrinsic material properties of the lasing medium. The laser linewidth measurements are corroborated by polarization spectroscopy studies as well as by analysis of hyperfine structure and cross-over resonances.
The invention, and subsequent development of external cavity quantum cascade lasers (EC-QCLs) has allowed relatively simple access to a wide spectral region for high resolution laser spectroscopy. In this Letter, we demonstrate the application of a broadly tunable 5.3 μm EC-QCL to absorption spectroscopy on the fundamental band of deuterium bromide. The large tuning range is utilised to record spectra allowing the hyperfine coupling constants to be determined for the v = 0 and v = 1 levels of both D79Br and D81Br. We present the first measurement to our knowledge of e2Qq(v=1)e2Qq(v=1) for D81Br, given a value of 460.5 ± 12 MHz.
A new class of IR coherent sources and IR frequency combs, that combine optical frequency-comb synthesizers (OFCSs) and optical parametric up/down-conversions, is already available and still progressing at a very fast pace. Peculiar features for IR radiation produced by difference-frequency-generation (DFG) set-ups or quantum-cascade lasers (QCLs) can he achieved when they are phase and frequency controlled by the OFCS. Indeed, their frequency is accurately known against the primary frequency standard and their linewidth is highly narrowed thanks to the transferred OFCS coherence even for laser sources whose frequencies are several THz apart. These features, together with their wide tunability and their small intensity fluctuations (down to the shot-noise limit), make these IR sources well suited for a wide range of applications, in particular for spectroscopic ones. Very high sensitivity for trace-gas detection has been achieved when combined with enhancement absorption techniques as high-finesse Fabry-Perot cavities or multipass cells. Moreover, the large number of fundamental ro-vibrational transitions of many stable and transient molecular species accessible with this spectrometers, make them particularly attractive for environmental applications, especially considering their compactness and ruggedness when a fiber-based set-up is chosen. Their unique capabilities in terms of achievable precision for absolute frequency measurements can he used to create a "natural" grid of secondary frequency standards of IR molecular absorptions, frequency measured with these high-resolution spectrometers. More important, we have directly generated an IR frequency comb around 3 mum by DFG Conversion of an OFCS. The generated comb can be employed both as a frequency ruler and a direct source for molecular spectroscopy.
Quantum cascade lasers (QCLs) are becoming well known as convenient and stable semiconductor laser sources operating in the mid- to long-wave infrared, and are able to be fabricated to operate virtually anywhere in the 3.5 to 25 micron region. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present, spanning at least three critical areas: national security, environmental monitoring and protection, and the early diagnosis of disease through breath analysis. There are many different laser-based spectroscopic chemical sensor architectures in use today, from simple direct detection through to more complex and highly sensitive systems. Many current sensor needs can be met by combining QCLs and appropriate sensor architectures, those needs ranging from UAV-mounted surveillance systems, through to larger ultra-sensitive systems for airport security. In this paper we provide an overview of various laser-based spectroscopic sensing techniques, pointing out advantages and disadvantages of each. As part of this process, we include our own results and observations for techniques under development at PNNL. We also present the latest performance of our ultra-quiet QCL control electronics now being commercialized, and explore how using optimized supporting electronics enables increased sensor performance and decreased sensor footprint for given applications.
We demonstrate the successful frequency stabilization of two QCLs to separate optical cavities using highly optimized servo loops based on the Pound-Drever-Hall technique. We present 5.6 Hz FWHM beat notes between these two systems that are stable for periods up to a second. This represents an effective reduction from the recently recorded free-running linewidth of these same lasers of 150 kHz of over 25,000.
Measurements of NO concentrations at sub-ppm levels in vehicle exhaust are needed for emissions certification of future ultra-low emission vehicles. We demonstrate a wavelength-modulation, laser-based, NO detection system suitable for this purpose. A quantum cascade distributed feedback laser (QC-DFB) operating continuous wave (cw) at ~100 K is frequency modulated at f = 10 kHz and locked to the center of a transition at ~1921 cm-1 in the fundamental band of NO. The demodulated signal at 2f of the beam passing through the sample cell directly measures the NO concentration. The cell is a multipass Herriott-type with a 100-m path length. Doppler broadening, pressure broadening, and unresolved Λ doubling combine to yield a pressure for optimum sensitivity of 100 torr and a modulation amplitude of ~600 MHz. A flowing gas system is used to avoid problems with adsorption and desorption of NO from the cell walls. The reduced pressure eliminates interference from other gas species. Detection of NO concentrations in the few parts-per-billion (ppb) range is demonstrated in diluted exhaust-gas bag samples collected in the vehicle certification process.
Summary form only given. Some of the applications of stabilized laser sources (mid-IR sub-Doppler spectroscopy, precision laser ranging) require reduction of the high frequency spectral noise components, resulting in fine narrowing and an increase in the coherence length of the laser. In order to achieve this, considerable servo bandwidth is required. For this reason, the phase transfer function behavior of the laser stabilization system is crucial, time delays and inductive effects being detrimental. In this paper, we report on our latest developments in QC laser stabilization, including some elegant electronics that have alleviated a number of common control theory issues including resonances and variability in the transfer function
In many examples of the use of mid-infrared quantum cascade (QC) lasers for gas detection or process monitoring, an assumption is made that their use is an obvious extension of tuneable diode laser spectroscopy. We wish to show that making such an assumption is not necessarily justified when the frequency sweep rate is rapid, as is down-chirped QC laser infrared radiation. This is demonstrated via a series of experiments designed to investigate the physics of the interaction of chirped infrared laser radiation with low pressure gases. The unusual signals, which characterise the rapid passage of the down-chirped radiation through a low pressure gas, are due to two main effects, the laser sweep rate, and the long path length of the refocusing cells used. The sweep rate of the laser frequency may be faster than the inter-molecular collision frequency, allowing the build up of a strong molecular alignment within the gas. The long optical path lengths in the refocusing absorption cells, used to facilitate sensitive detection of trace gases, allow the build up of a large macroscopic polarisation within the gas cell. We give examples of this behaviour in molecules with large transition dipole moments, ammonia and nitrous oxide, and with a very small one OCO. We also outline the use of Maxwell–Bloch calculations to investigate the origins of this behaviour, and hence to define operating conditions where the concentration of trace molecules may be determined.
Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance.
The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.
A continuous wave quantum cascade laser (cw-QCL) operating at 10 μm has been used to record absorption spectra of low pressure samples of OCS in an astigmatic Herriott cell. As a result of the frequency chirp of the laser, the spectra show clearly the effects of rapid passage on the absorption line shape. At the low chirp rates that can be obtained with the cw-QCL, population transfer between rovibrational quantum states is predicted to be much more efficient than in typical pulsed QCL experiments. This optical pumping is investigated by solving the Maxwell Bloch equations to simulate the propagation of the laser radiation through an inhomogeneously broadened two-level system. The calculated absorption profiles show good quantitative agreement with those measured experimentally over a range of chirp rates and optical thicknesses. It is predicted that at a low chirp rate of 0.13 MHz ns(-1), the population transfer between rovibrational quantum states is 12%, considerably more than that obtained at the higher chirp rates utilised in pulsed QCL experiments.
Femtosecond-laser-based optical frequency comb synthesizers (OFSs) can play a fundamental role in precision spectroscopy and
laser frequency control in the infrared (IR), as has happened in the visible and near-IR regions of the spectrum. Increasing
demand for precise frequency measurements in the IR has resulted in a number of ideas for IR frequency synthesis (IFS) with
visible combs. IR frequency synthesis and measurement require narrow linewidth, well-controlled and often widely tunable laser
sources as well as high-accuracy frequency references. In this paper we review the state of the art of IR metrology following
the advent of the OFS. In particular, we describe our system which uses a nonlinear difference-frequency-generated (DFG) IR
source combined with an OFS to develop a widely tunable IR laser source with very narrow linewidth and with a frequency directly
traceable to the Cs primary standard. The frequencies of saturated absorption spectra belonging to the intense molecular ro-vibrational
transitions in this region can be measured with our OFS-referenced DFG source. An unprecedented dense grid of secondary frequency
standards can be created with this DFG-OFS combination.
In this paper we investigate the performance of quantum cascade (QC) lasers for high frequency modulation spectroscopy, particularly
using frequency modulation (FM) and two-tone (2T) techniques. The coupling of the rf signal to the QC laser through the cryostat
is studied in detail as well as the noise contributions of both the detector and the laser source to the final spectra. The
experimental traces are obtained by spectroscopy on low-pressure N2O and CH4 gases at 8.0μm and 7.3μm wavelength, respectively, and reproduce the line profiles predicted by theory. As a preliminary
result, an enhancement of a factor six is measured with respect to direct absorption line recording.
Using a low power, rapid (nsec) pulse-modulated quantum cascade (QC) laser, collective coherent effects in the 5 μm spectrum of nitric oxide have been demonstrated by the observation of sub-Doppler hyperfine splitting and also Autler-Townes splitting of Doppler broadened lines. For nitrous oxide, experiments and model calculations have demonstrated that two main effects occur with pulse-modulated (chirped) quantum cascade lasers: free induction decay signals, and signals induced by rapid passage during the laser chirp. In the open shell molecule, NO, in which both Λ-doubling splitting and hyperfine structure occur, laser field-induced coupling between the hyperfine levels of the two Λ-doublet components can induce a large ac Stark effect. This may be observed as sub-Doppler structure, field-induced splittings, or Autler-Townes splitting of a Doppler broadened line. These represent an extension of the types of behaviour observed in the closed shell molecule nitrous oxide, using the same apparatus, when probed with an 8 μm QC laser.
Summary form only given. The ultra-thin silicon-on-sapphire process uses 100 nm silicon on top of fully insulating, optically transparent synthetic sapphire. The process has good thermal conductivity properties and thermal expansion coefficients well-matched to flip-chip-bonded GaAs optical devices. It is one example of a silicon-on-sapphire CMOS process with great potential for photonic integration. We present the design, simulation, fabrication and test of UTSi chips containing several test circuits and subsystems useful for optical communications and interconnections.
A room temperature external grating cavity (EGC) quantum cascade laser (QCL) is characterized using saturation spectroscopy of N O <sub>2</sub> . The presence of two strong EGC QCL waveguide modes is evident from the saturation spectra. A linewidth of 21 MHz for each EGC-QCL mode is measured from the width of the saturation peak at 10 mTorr pressure of N O <sub>2</sub> .
A supersonic free-jet spectrum of the ν4 band of CF3Cl has been measured using a quantum cascade laser system. Those measurements were combined with a low temperature (−67 °C) FTS spectrum of the region 1060–1260 cm−1 and with room temperature FTS measurements down to 400 cm−1 to give improved values for the rovibrational constants for the ν1, ν2, ν3, 2ν3, 2ν5, ν4, and ν5 states of CF335Cl and CF337Cl. The principal perturbation found by earlier investigators in the ν1 band is treated as a very weak Coriolis interaction at several avoided crossings of the rotational levels of the ν1 state and the 2ν5 state with kl < 0. None of the other vibrational states showed any signs of perturbations. With these new measurements we now have high resolution data on all of the fundamental vibrational states except ν6.
Rotational vibrational. ne structure and transition dipole moment of NO(2) is measured using Doppler free saturation spectroscopy with an external grating cavity quantum cascade laser (QCL). The QCL wavelength is calibrated using a 310 cm long internally coupled Fabry-Perot interferometer. We obtain a frequency splitting of 139.68 +/- 0.06 MHz (0.0047 cm(-1)) between the spin doublets P(4)(+/-) (17) of 000 ? 001 transition of NO2. The resolution of the QCL based saturation spectrometer is limited by the QCL line-width of similar to 3.99 MHz (similar to 0.00013 cm(-1)) deduced from the half width of the Lamb dips. The Lamb dip spectroscopy is utilized to obtain a vibrational dipole moment of similar to 0.37 Debye for the P(4)(+/-) (17) transitions.
In the short space of 15 years since their first demonstration, quantum cascade lasers have become the most useful sources of tunable mid-infrared laser radiation. This Letter describes these developments in laser technology and the burgeoning applications of quantum cascade lasers to infrared spectroscopy. We foresee the potential application of quantum cascade lasers in other areas of chemical physics such as research on helium droplets, in population pumping, and in matrix isolation infrared photochemistry.
We report on recent development of IR spectrometers based on non-linear optical generation and quantum-cascade lasers. Frequency stabilization and referencing to optical frequency-comb synthesizers is described. Their characteristics for spectroscopic as well as metrological applications are pointed out. The potential of the combination of mid-IR sources with enhancement cavities for high-resolution and sensitive measurements of gas spectra is illustrated. Finally, we describe wave-front engineering of long wavelength beams for spatial control and imaging applications.
We report line width measurements of a quantum cascade distributed feedback laser by a heterodyne experiment. At currents slightly above threshold and a laser output power higher than 1 mW, the full width at half maximum of the beat signal was narrower than 0.5 MHz, which gives us an upper limit of the laser line width. As reference laser we used a carbon monoxide laser. Both lasers were operating unstabilized in continuous wave mode emitting light at about 5.2 μm.
We review the use of both pulsed and continuous wave quantum cascade
lasers in high-resolution spectroscopic studies of gas phase species. In
particular, the application of pulsed systems for probing kinetic
processes and the inherent rapid passage structure that accompanies
observations of low-pressure samples using these rapidly chirped devices
are highlighted. Broadband absorber spectroscopy and time-resolved
concentration measurements of short-lived species, respectively
exploiting the wide intrapulse tuning range and the pulse temporal
resolution, are also mentioned. For comparison, we also present recent
sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using
a continuous wave external cavity quantum cascade laser system. Using
this methodology the stability and resolution of this source is
quantified. We find that the laser linewidth as measured via the
Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates,
but decreases to 1.3 MHz as the laser scan rate is increased such that
the transition is observed at 30 kHz. Using this source, wavelength
modulation spectroscopy of NO is presented.
A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry-Perot quantum cascade laser (QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene (C(60)). As a first test of the system, high resolution spectra of the nu(8) vibrational band of CH(2)Br(2) have been acquired at approximately 1197 cm(-1). To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of CH(2)Br(2) were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of 1.4 x 10(-8) cm(-1). Spectral simulations of the band indicate that the supersonic source produces rotationally cold (approximately 7 K) molecules.
The quantum-cascade laser can be used as an infrared source for a small portable photoacoustic trace gas detector. The device that we describe uses a quantum-cascade laser without collimating optics mounted inside an acoustic resonator. The laser is positioned in the center of a longitudinal resonator at a pressure antinode and emits radiation along the length of the resonator exciting an axially symmetric longitudinal acoustic mode of an open-ended cylindrical resonator. Experiments are reported with an 8-microm, quasi-cw-modulated, room-temperature laser used to detect N2O.
Quantum cascade lasers (QCLs) are a relatively new type of semiconductor laser operating in the mid- to long-wave infrared. These monopolar multilayered quantum well structures can be fabricated to operate anywhere between 3.5 and 20 microm, which includes the molecular fingerprint region of the infrared. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present. Frequency stabilization and injection locking increase the utility of QCLs. We present results of locking QCLs to optical cavities, achieving relative linewidths down to 5.6 Hz. We report injection locking of one distributed feedback grating QCL with light from a similar QCL, demonstrating capture ranges of up to +/-500 MHz, and suppression of amplitude modulation by up to 49 dB. We also present various cavity-enhanced chemical sensors employing the frequency stabilization techniques developed, including the resonant sideband technique known as NICE-OHMS. Sensitivities of 9.7 x 10(-11) cm(-1) Hz(-1/2) have been achieved in pure nitrous oxide.
The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.
We report the application of a cw distributed feedback quantum cascade laser to Lamb-dip spectroscopy of CO2 at 4.3 microm. With the laser operating in the free-running mode, we observed the sub-Doppler profile of the P(28) line of the (0,1(1),0)->(0,1(1),1) hot band by implementing a pump-probe scheme and using wavelength modulation spectroscopy for highly sensitive detection of saturated absorption signals. We investigated the main limitations to the observation of a narrow resonance, with particular attention to the effect of the laser current noise. We determined the intrinsic laser emission width, which was found to be approximately 3.4 MHz (FWHM) for an observation time of approximately 200 ms.
A successful two-photon spectroscopy experiment using combined C${\mathrm{O}}_{2}$ and diode lasers operating in the 10-$\mu${}m region is reported. Beams from the two lasers were passed in nearly opposite directions through a sample cell while the intensity of the transmitted diode laser beam was monitored as a function of its frequency. Doppler-free two-photon absorption signals were observed for the rovibrational transitions of N${\mathrm{H}}_{3}$, $T({v}_{2},J,K)=({2}^{$-${}},7,7)$\leftarrow${}({0}^{$-${}},7,7) \mathrm{and} ({2}^{$-${}},1,1)$\leftarrow${}({0}^{$-${}},1,1)$.
We present an optically stabilized lead-salt diode-laser system which is the nucleus of a very-high-resolution instrument for sub-Doppler molecular spectroscopy in the mid-infrared. By application of external optical feedback, we have narrowed the diode-laser linewidth by two orders of magnitude, yielding a spectral width of less than 200 kHz. The diode- laser frequency is stabilized and controlled via the external reflector by variable-frequency offset-locking the diode-laser to a CO laser frequency. This substantial improvement in the spectral properties enabled us to perform a Lamb-dip experiment on a carbonyl sulfide (OCS) absorption line near 1985 cm–1. We were able to detect a saturated dispersion signal at low pressure (5 Pa) with a signal-to-noise ratio of several thousand. The present paper describes the unique features of the optically stabilized tunable diode-laser system and its use as a spectroscopic tool for sub-Doppler applications.
This new calibration atlas is based on frequency rather than wavelength calibration techniques for absolute references. Since a limited number of absolute frequency measurements is possible, additional data from alternate methodology are used for difference frequency measurements within each band investigated by the frequency measurements techniques. Data from these complementary techniques include the best Fourier transform measurements available. Included in the text relating to the atlas are a description of the heterodyne frequency measurement techniques and details of the analysis, including the Hamiltonians and least-squares-fitting and calculation. Also included are other relevant considerations such as intensities and lincshape parameters. A 390-entry bibliography which contains all data sources used and a subsequent section on errors conclude the text portion.
The primary calibration molecules are the linear triatomics, carbonyl sulfide and nitrous oxide, which cover portions of the infrared spectrum ranging from 488 to 3120 cm⁻¹. Some gaps in the coverage afforded by OCS and N2O are partially covered by NO, CO, and CS2. An additional region from 4000 to 4400 cm⁻¹ is also included.
The tabular portion of the atlas is too lengthy to include in an archival journal. Furthermore, different users have different requirements for such an atlas. In an effort to satisfy most users, we have made two different options available. The first is NIST Special Publication 821, which has a spectral map/facing table format. The spectral maps (as well as the facing tables) are calculated from molecular constants derived for the work. A complete list of all of the molecular transitions that went into making the maps is too long (perhaps by a factor of 4 or 5) to include in the facing tables. The second option for those not interested in maps (or perhaps to supplement Special Publication 821) is the complete list (tables-only) which is available in computerized format as NIST Standard Reference Database #39, Wavelength Calibration Tables.
Semiconductor lasers are widely used in modern life. In telecommunications they send signals for thousands of kilometres along optical fibres. In consumer electronics, semiconductor lasers are used to read the data on compact disks and CD-ROMs. Other applications include laser printers and laser pointers. Although they are just the size of a grain of salt, typically a few microns in cross-section and a few hundred microns long, semiconductor lasers are an integral part of the modern world.
Substantial narrowing of the Doppler profile of molecular absorption lines is achieved in an effusive molecular beam from a capillary array nozzle probed with a broadly tunable diode laser propagating transverse to the flow. Hitherto unresolved Lambda-doublet splittings in the fundamental v = 1
Saturation of molecular transitions using a tunable diode laser has been demonstrated for the first time using a standing-wave-cavity configuration with the laser beam focused at the sample. Observed saturation effects in NH3 transitions near 888 kaysers include sub-Doppler (Lamb-dip) resonances at the line center.
Quantum-cascade distributed-feedback lasers with high-power, continuous-wave (cw), tunable, single-mode emission are reported. The emission wavelengths are near 5.2 and 7.95 μm. The lasers are operated at liquid-nitrogen temperature and above. A maximum output power of >100 mW is obtained per facet at 80 K for both wavelengths, which is the result of careful positioning of the peak gain with respect to the Bragg wavelength. Continuous tuning with either heat-sink temperature or cw current is demonstrated. The tuning coefficients are 0.35 nm/K (5.2 μm) and 0.51 nm/K(7.95 μm) for thermal tuning and vary from 20 to 40 nm/A for tuning with current. The lasers are being used in high-resolution and high-sensitivity gas-sensing applications.
Double resonance experiments on NH3 are described in which a CO laser pumps a 6 μm ν4 transition while a tunable diode laser probes a 10 μm ν2 transition having a common lower level. Four different combinations of pump–probe transitions are studied. The CO laser is Lamb‐dip stabilized on the pump transitions, which are tuned into coincidence with it using a precision intracavity Stark cell. The pump and probe beams overlap colinearly in the Stark cell. The double resonance signals appear as narrow transmission peaks on the diode laser scans. The narrowest observed widths are ≂3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that the diode laser contributes less than 1 MHz to the widths. Resonances due to velocity‐preserving but state‐changing collisions are seen. Asymmetries between co‐ and counterpropagating linewidths are shown to arise from a combination of field inhomogeneity and coherent narrowing effects. The data are recorded using a computer‐aided, rapid‐scan, digital signal averaging technique.
The fundamental, quantum phase noise limited Lorentzian linewidth was directly measured from the beat‐note spectra generated by heterodyning PbS 1 -x Se x diode lasers with a stable CO gas laser. The experimental results were matched by calculated theoretical line profiles. Linewidths as narrow as 22 kHz full width at half‐maximum power were observed.
We describe in this paper the modifications, improvements, and enhancements to the HITRAN molecular absorption database that have occurred in the two editions of 1991 and 1992. The current database includes line parameters for 31 species and their isotopomers that are significant for terrestrial atmospheric studies. This line-by-line portion of HITRAN presently contains about 709,000 transitions between 0 and 23,000 cm-1 and contains three molecules not present in earlier versions: COF2, SF6, and H2S. The HITRAN compilation has substantially more information on chlorofluorocarbons and other molecular species that exhibit dense spectra which are not amenable to line-by-line representation. The user access of the database has been advanced, and new media forms are now available for use on personal computers.
Frequency stabilization of mid-IR quantum cascade (QC) lasers to the kilohertz level has been accomplished by use of electronic servo techniques. With this active feedback, an 8.5-mu m QC distributed-feedback laser is locked to the side of a rovibrational resonance of nitrous oxide (N2O) at 1176.61 cm(-1). A stabilized frequency-noise spectral density of 42 Hz/root Hz has been measured at 100 kHz; the calculated laser linewidth is 12 kHz. (C) 1999 Optical Society of America OCIS codes: 000.2170, 120.5050, 120.4820, 140.5960, 000.2170.
Lasing characteristics were evaluated for distributed-feedback quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the QC lasers' suitability for use in high-resolution spectroscopic applications, including Doppler-limited molecular absorption and pressure-limited lidar applications. By use of a rapid-scan technique, direct absorbance measurements of nitric oxide (NO) and ammonia (NH>3) were performed with several QC lasers, operating at either 5.2 or 8.5 textmum. Results include time-averaged linewidths of better than 40 MHz and long-term laser frequency reproducibility, even after numerous temperature cycles, of 80 MHz or better. Tuning rates of 2.5 cm-1 in 0.6 ms can be easily achieved. Noise-equivalent absorbance of 3 texttimes 10-6 was also obtained without optimizing the optical arrangement.
The and transitions of the nitric oxide fundamental are reported with an experimental line-width (F.W.H.M.) of 13.9 MHz. The spectra show well resolved hyperfine structure from the 14N nuclear spin and are in excellent agreement with predictions based on the microwave spectrum of nitric oxide. It is concluded that the major contributor to the observed line-width is the PbSxSe1−x diode laser used to observe the spectra.
Laboratory Fourier transform absorption spectroscopy has been used to obtain new measurements of spectral line parameters in the P and R branches of the two allowed 1-0 subbands of NO. Parameters measured are the self-broadened widths and absolute strengths at room temperature and the N2-broadened widths over the temperature range 213-296 K. From these data a band-strength at 296 K (4.45 * 10-17 cm-1/mol cm-2) and an average temperature exponent of N2-broadened linewidths (-0.71) have been derived. The J dependence of the temperature exponent was examined and found to be insignificant compared to experimental error.
Results are given for a Stark-tuned double-resonance experiment, using a CO-laser pump and a diode-laser probe. The CO laser, operating on the 13-12 P(15) line at 1775.2588 cm(-1), is locked by Lamb-dip stabilization to one of the Stark components of the a(R)R(9, 9) nu(4) line of NH(3). The diode laser probes the aQ(9, 9) nu(2) line at 921.255 cm(-1), revealing a complex spectrum of sub-Doppler features, the narrowest of which are 5.3 +/- 0.3 MHz wide. As well as the resonances associated with population depletion of the common lower levels, we see line-narrowing effects that are due to two-quantum Raman-type processes and collision-induced resonances arising from state-changing collisions that preserve the molecular velocities. The zero-field a(R)R(9, 9) nu(4) line is established to be 264.1 +/- 5.0 MHz above the CO-laser line.
A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.
Quantum-cascade distributed-feedback lasers with high-power, continuous-wave (cw), tunable, single-mode emission are reported. The emission wavelengths are near 5.2 and 7.95 mum. The lasers are operated at liquid-nitrogen temperature and above. A maximum output power of >100 mW is obtained per facet at 80 K for both wavelengths, which is the result of careful positioning of the peak gain with respect to the Bragg wavelength. Continuous tuning with either heat-sink temperature or cw current is demonstrated. The tuning coefficients are 0.35 nm/K (5.2 mum) and 0.51 nm/K(7.95 mum) for thermal tuning and vary from 20 to 40 nm/A for tuning with current. The lasers are being used in high-resolution and high-sensitivity gas-sensing applications.
Quantum cascade lasersHigh-power, cw, current-tunable, single-mode QC-DFB lasers at 5.2 and 7.95 �m
- F Capasso
- C Gmachl
- D L Sivco
- A Y Cho
- C Gmachl
- F Capasso
- A Tredicucci
- D L Sivco
- J N Baillargeon
- A L Hutchinson
- A Y Cho