Intracavity Absorption Research Articles

Magneto-induced `bleaching' of an absorbing medium in an anisotropic laser cavity

We describe experiments and the corresponding theory of a helium-neon laser containing an optical absorber cell with pure neon, a Faraday rotator and a partial polarizer. The absorber can be modulated. Additionally, a longitudinal magnetic field H induces rotation of the polarization plane of light in the absorber. Modulated output power of the laser is observed. We find good qualitative agreement of the measured data with the theoretical expectation. Under certain conditions the laser system demonstrates unusual behaviour: the output modulation vanishes for a specific choice of H, i.e. the laser becomes insensitive to the presence of the intracavity absorber. The possible spectroscopic application of this effect is briefly discussed.

Diode-pumped broadband vertical-external-cavity surface-emitting semiconductor laser applied to high-sensitivity intracavity absorption spectroscopy

A diode-pumped broadband multiple-quantum-well vertical-external-cavity surface-emitting semiconductor laser has been developed for high-sensitivity intracavity laser-absorption spectroscopy. The semiconductor structure design has been optimized so as to provide maximum laser-emission bandwidth and wavelength tunability. The laser has a 100-mW threshold of continuous room-temperature operation, and it can be tuned within 25 nm around its design wavelength (980 nm). A detection limit lower than 10-10 per centimeter of absorption path has been achieved, given ∼3×10-11 cm-1 Hz-1/2. Its spectro-temporal dynamics has been studied in the time range from a few microseconds to ∼1 s. No evidence of nonlinear mode interactions, which in many cases limit the sensitivity, has been observed. We have also shown that with a cavity length reduced to 2.5 cm, the laser is very attractive as a tunable single-frequency source owing to its stable operation in a single TEM00 mode at a pump power of up to 1 W.

Absence of the laser output response to modulation of the optical density of the intracavity absorber

It is found that a laser with an anisotropic cavity containing an absorbing medium may exhibit no response of the output radiation intensity to the modulation of the optical density of the absorbing medium in the presence of a magnetic field. The conditions when this effect may be observed are determined, and its possible applications are briefly discussed.

Refined Investigation of the Overtone Spectrum of Nitrous Oxide

Following our previous work [A. Campargue, D. Permogorov, M. Bach, M. Abbouti Temsamani, J. Vander Auwera, M. Herman, and M. Fujii, J. Chem. Phys. 103, 5931–5938 (1995)], we reinvestigated the overtone spectrum of nitrous oxide (14N216O) using intracavity laser absorption spectroscopy, observing seven new bands in the spectral range 10 650–12 200 cm−1 and improving the signal-to-noise ratio in two others previously reported using Fourier transform spectroscopy. The earlier FT line positions, in the range 6500–11 000 cm−1, have been calibrated using the 3–0 vibration–rotation line wavenumbers in 12C16O as references. A previously unreported hot band is spotted on the FT spectrum, at 7229 cm−1. Vibration–rotation parameters are provided for all newly analyzed bands and significantly improved for most previously reported FT bands by accounting for more lines in the fitting procedure than earlier. All bands are vibrationally assigned in agreement with theoretical predictions. A table listing all levels above 6500 cm−1 reported in the literature is provided.

Frequency-switched heterodyne cavity ringdown spectroscopy

When the frequency of light coupled into a cavity is suddenly shifted, the radiation emanating from the input port of the previously excited cavity can beat with the reflection of the frequency-shifted input on the surface of a photodetector. When the beat frequency is stable, the time decay of the resulting optical heterodyne signal can be used to measure intracavity absorption spectra with near quantum-limited sensitivity.

Cavity ringdown heterodyne spectroscopy: High sensitivity with microwatt light power

We present an ac heterodyne technique in cavity ringdown spectroscopy that permits 1310 absorption sensitivity with microwatt-level light power. Two cavity modes, one probing the empty cavity and the other probing intracavity absorption, are excited simultaneously but with their intensities temporally out of phase, with one mode decaying and the other rising. Heterodyne detection between the two modes reveals the dynamic time constants associated with the empty cavity and the additional intracavity gas absorption. The method offers a quick comparison between the onand off-resonance information, a prerequisite to reaching the fundamental shot-noise limit. This simple and yet important improvement of cavity ringdown spectroscopy should lead to the enhanced performance of a wide range of applications.

Diode-pumped broadband vertical external cavity surface emitting semiconductor lasers. Application to high sensitivity intracavity absorption spectroscopy

Summary form only given. Absorption spectroscopy is widely used in the laboratory and for in situ measurements. One of the most important issues of any monitoring technique, as important as the sensitivity, is the ability to provide absolute absorption coefficients without the need for complicated and cumbersome calibration procedures. Intracavity-laser-absorption-spectroscopy (ICLAS) is one of the most sensitive techniques capable of providing this absolute information. In the ICLAS technique an absorbing medium with absorption lines narrower than the broadband spectrum of a laser is placed inside the laser cavity. The absorption lines follow the Lambert-Beer law, with an equivalent path length of L/sub eq/=ctg, where the generation time t/sub g/ is the time from the beginning of the laser generation to the moment of observation, and c is the velocity of light. Any type of laser with a homogeneously broadened gain medium can be used. ICLAS allows extreme sensitivity, but, up to now, it has been limited mainly to the 0.6. 1 /spl mu/m range, with expensive and bulky systems.

Application of the three-mode operation of a broad-band laser in intracavity dispersion frequency-domain spectroscopy

The beats of a three-mode dye laser, containing a component with a frequency close to the difference between neighbouring mode spacings, the technical fluctuations of which cancel out, are considered. The aim is to develop a laser spectroscopic method with a detection limit of the order of 10-10 — 10-11 cm-1 Hz-1/2, determined by the natural frequency fluctuations and by the spectral resolution limited by the homogeneous width of the resonance line. Use was made of the unequal changes in two mode spacings of an equidistant three-mode spectrum, caused by the dispersion of the absorption line of an intracavity absorber, and of the detection of the resulting difference frequency.

Spectral dynamics of multimode Nd 3+- and Yb 3+-doped fibre lasers with intracavity absorption

Diode-pumped Nd 3+- and Yb 3+-doped multimode fibre lasers with external cavities allow sensitive measurements of intracavity absorption in the spectral ranges of 0.9–0.945 μm and 1.03–1.15 μm. The sensitivity is characterized by the effective absorption path length and is derived from the spectral dynamics of the lasers with intracavity atmospheric absorption, and calibrating the emission spectra with HITRAN database. The highest sensitivity is achieved with a cw Nd 3+-doped fibre laser; it corresponds to a 140 km effective path length of absorption. Since the achieved sensitivity decreases only weakly upon the pump rate, it seems to be restricted by the joint action of Rayleigh scattering and nonlinear mode coupling. Both multimode fibre lasers may be made the base of sensitive portable multicomponent gas detectors.

HIGH RESOLUTION FOURIER-TRANSFORM INTRA-CAVITY LASER ABSORPTION SPECTROSCOPY: THEORY AND APPLICATION

The theory of detecting the Intra-cavity Laser Absorption Spectroscopy by utilizing a continuous-scan high-resolution Fourier-transform interferometer is introduced.The method to record the position and intensity of absorption lines with the Fourier-transform Intra-cavity Laser Absorption Spectrometer is discussed.By recording the atmospheric water absorption in the region of 12450—12700cm-1,the reliability of this method is investigated.And the technique is applied to record the νOD = 5 stretching overtone of the deuterated water(D2O and HDO).

The Near-Infrared Transition of CuCl Observed by Intracavity Laser Spectroscopy

The near-infrared electronic transition of CuCl, occurring in the region of 745 nm, was recorded using intracavity laser absorption spectroscopy. The (0, 0), (1, 1), and (2, 2) vibronic bands were analyzed, and from this the molecular constants for the two electronic states were derived. Originally assigned as A′ 3Σ+–X1Σ+, we have confirmed that this transition does not connect to the ground state, but occurs between two unknown excited states. The excited CuCl molecules were produced in a copper hollow cathode, operated using argon and a small amount of CCl4. Line positions were referenced to iodine spectra observed from a heated extracavity cell using the broadband spectral output of the intracavity laser as the light source.

High resolution absorption spectroscopy of the ν1=2–6 acetylenic overtone bands of propyne: Spectroscopy and dynamics

The rotationally resolved nν1 (n=2–6) overtone transitions of the CH acetylenic stretching of propyne (CH3–C≡C–H) have been recorded by using Fourier transform spectroscopy (n=2), various intracavity laser absorption spectrometers (n=3, 4, and 6) and cavity ring down spectroscopy (CRDS) (n=5). The 2ν1, 3ν1, and 6ν1 bands exhibit a well-resolved and mostly unperturbed J-rotational structure, whose analysis is reported. The 5ν1 band recorded by pulsed CRDS shows an unresolved rotational envelope. In the region of 12 700 cm−1, an anharmonic interaction is confirmed between 4ν1 and 3ν1+ν3+ν5. The band at a higher wave number in this dyad exhibits a partly resolved K-structure, whose analysis is reported. The mixing coefficient of the two interacting states is determined consistently using different procedures. The 1/35 anharmonic resonance evidenced in the 4ν1 manifold induces weaker intensity borrowing from the 2ν1 and 3ν1 levels to the ν1+ν3+ν5 and 2ν1+ν3+ν5 level, respectively, which have been predicted and identified. Several hot bands around the 2ν1, 3ν1, and 3ν1+ν3+ν5 bands arising from the ν9=1 and ν10=1 and 2 bending levels are identified and rotationally analyzed, also leading to determine x1,9 [−20.3(3) cm−1], x1,10 [−1.7975(75) cm−1], and x3,10 [−6.56 cm−1]. The J-clumps of the P and R branches in the 6ν1 band at 18 499 cm−1 show a Lorentzian homogeneous profile mostly J-independent with an average full width at half maximum (FWHM) of 0.17 cm−1, attributed to arising from the intramolecular vibrational energy redistribution towards the bath of vibrational states. A detailed comparative examination of the fine structure in all investigated nν1 (n=2 to 7) overtone bands and the similar behavior of the cold and hot bands arising from ν10=1 definitively suggests that a highly specific low-order anharmonic coupling, still unidentified, dominates the hierarchy of interaction mechanisms connecting the nν1 levels to the background states.

Mathematical modeling of TE CO 2 laser with SF 6 as a saturable absorber

A mathematical model describing the dynamic emission of a single mode TE CO 2 laser with saturable absorber has been adapted. A six-temperature model has been used to describe the amplifying medium, while a four-coupled energy level is used to describe the selective absorbing medium. The suggested mathematical model allows the investigation of the effects of the intracavity absorber on the mode characteristics of the TE CO 2 laser and, moreover, the study of the effect of the laser input parameters on the output laser pulse. The model simulates the passive Q-switch in both low- and high-pressure cases in the absorbing medium. In addition, numerical solutions of a non-linear rate equation system of the suggested model are quantitatively discussed. The solutions describe the photon number density, the population inversion and the energy transfer processes of amplifying and absorbing media.

Identification of Triplet States in Carotenoids by Intracavity Absorption Spectroscopy and Measurements of Delayed Fluorescence.

Owing to the low quantum yield of phosphorescence, triplet states of carotenoids have been very difficult to identify. These states can be characterized by intracavity absorption spectroscopy, which allows the direct measurement of the spin-forbidden singlet-triplet transitions in low concentrated solutions, and by delayed fluorescence measurements.

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sen- sitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorp- tion. The sensitivity of measurements of intracavity absorp- tion grows with the laser pulse duration. The ultimate sen- sitivity obtained with a cw laser is set by various perturba- tions of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dy- namics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70 000 km effective length of the absorption path. The ultimate spec- tral resolution is determined by the spectral width of mode emission, which is 0: 7H zin this dye laser. High sensitiv- ity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers.

Highly sensitive detection of gaseous impurities by intracavity laser spectroscopy based on a Co:MgF2 laser

The intracavity absorption spectra of CO2 and H2O in the range 2.03 — 2.05 μm, as well as NH3 and CH4 in the range 2.23 — 2.26 μm, were obtained with the aid of a Co:MgF2 laser. It was found that an intracavity laser spectrometer based on a Co:MgF2 laser emitting for 1 ms can be used to detect many impurity gases in the atmosphere at a concentration level of (1 — 40) × 10-9.

Cw Integrated cavity output spectroscopy

A new approach is described in which continuous, narrow band laser sources are employed with the recently developed integrated cavity output spectroscopy technique to obtain sensitive, quantitative absorption spectra in a simple experimental configuration. Absorption data obtained with cw-ICOS are related to the classical Fabry–Perot intracavity absorption model, which describes why the intracavity absorption is enhanced. A method of continuously injecting cw laser light into the cavity is described, as is a simple means of interpreting the ICOS data to extract accurate absorption intensities. Absorption spectra of vibrational combination bands of CO 2 and H 2O in the 1.3 μm region are presented.

High-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasers

We report the demonstration of high-sensitivity intracavity laser absorption spectroscopy with multiple-quantum-well vertical-external-cavity surface-emitting semiconductor lasers (VECSEL's). A detection limit of 3 x 10(-10) cm (-1) has been achieved. The spectrotemporal dynamics of a VECSEL in the 1030-nm wavelength region has been studied. The laser was operating cw at room temperature, with a baseline signal-to-noise ratio as high as 400. The laser was optically pumped with a threshold as low as 80 mW and was broadly tunable over a spectral range of approximately 75 nm .

Attenuated total internal reflection spectroscopy with an intracavity laser absorption spectrometer

A new type of intracavity laser absorption spectrometer (ICLAS) which employs an intracavity prism in total internal reflection (TIR) configuration is presented. We report a theoretical background and a full characterisation of the spectrometer by measuring absorption spectra of NO 2 and I 2 vapour at room temperature in the 0.6 μm spectral region. The ICLAS spectra measured in the attenuated TIR configuration correspond to bulk absorption with an equivalent number of reflections of 2×10 4. They are identical to the conventional gas phase absorption spectra with no appreciable contributions from (surface) adsorbed molecules.

New aspects concerning the energy transfer in carotenoids by measuring intracavity absorption spectra and delayed fluorescence

The first excited singlet state and the lower energetic triplet states of carotenoids are considered to be involved in the light-harvesting as well as in the photochemical protection of cells, respectively. For this reason, the symmetry-forbidden S 0–S 1 (1 1A g–2 1A g) transitions and the multiplicity-forbidden S 0–T 2 (1 1A g–2 3A g) transition of the model carotenoid 8,13-dimethyl-2,2,19,19-tetramethoxy-icosa-4,6,8,10,12,14,16-heptaene-3,18-dione were investigated by intracavity absorption spectroscopy from low-concentrated ethanolic solutions. Both transitions are shaped by promoting modes caused by Herzberg–Teller coupling and the sequence of these modes allows the precise determination of the non-visible S 0–S 1 (0–0)- and S 0–T 2 (0–0)-transitions. The assignments of the singlet–triplet transitions were additionally supported by measuring delayed fluorescence from crystalline samples by directly exciting vibronic triplet states. The vibronic coupling is promoted by C–H bending vibrations of the chain and mainly by deformation modes of the terminating groups of the carotenoid.