Intracavity Absorption Research Articles

Intracavity gain and absorption dynamics of hybrid modelocked semiconductor lasers using multiple quantum well saturable absorbers

Time resolved intracavity gain, saturable absorption dynamics, intracavity intensity pulse profiles, and their corresponding spectrograms, were measured in an external cavity hybrid mode-locked semiconductor diode laser. These measurements were performed to obtain fundamental information of the mode-locking dynamics and to determine their role in the pulse shaping and chirping dynamics. The results of these experiments show that the integrating nonlinearity associated with gain depletion, coupled with group velocity dispersion, leads to asymmetric intensity pulse profiles with predominantly cubic temporal phase, while saturable absorption coupled with group velocity dispersion tends to linearize the chirp. Exploitation of these dynamics may allow researchers to generate optical pulses with higher peak intensities than previously reported.

Intracavity laser absorption spectroscopy detection of singlet CH 2 radicals in hydrocarbon flames

The spectrum of methylene, the CH 2 radical, in the singlet electronic state was detected with a high signal/noise ratio in methane-oxygen-nitrogen flames using intracavity laser absorption spectroscopy. The spectra were recorded at different pressures including first measurements at atmospheric pressure. The maximum concentration of the singlet CH 2 radical in a 30 Torr flat flame was estimated based on literature data from laser photolysis experiments done at room temperature. The value of 2 × 10 13 cm −3 seems to be higher than that predicted by the model calculations. The dependence of the calculated maximum concentration of singlet CH 2 on the equivalence ratio is in qualitative agreement with measured values.

Temperature measurements in flames using water molecule overtone . spectra detected by intracavity laser absorption spectroscopy

Overtone absorption spectra of water molecules were recorded with intracavity laser absorption spectroscopy (ICLAS) based on a Ti:Sapphire laser with atmospheric pres- sure methane=air or hydrogen=air flame was placed into the cavity of the Ti:Sapphire laser. Though the flame occupied a small part of the cavity, substantial changes in the water spectrum were observed and new lines appeared. This sug- gests the possibility of measuring temperature in flames using an ICLAS spectrum of the water. Computer simulations based on the water spectrum lines assigned in the literature showed that a sensitivity of about ten degrees can be expected. The use of lines with higher rotational quantum numbers could substantially improve the sensitivity.

Cavity ringdown laser absorption spectroscopy detection of formyl (HCO) radical in a low pressure flame

The formyl radical has been detected via the Ã-X̃ system in a low pressure CH 4/N 2/O 2 flame using cavity ringdown laser absorption spectroscopy. The direct absorption data obtained in these initial studies allows determination of the absolute HCO concentration in the flame of ca. 2.1(±0.9) × 10 13 mol cm −3, which is in excellent agreement with flame chemistry simulations. The extrapolated detection limit at 300 K of ≈ 1.4(±0.6) × 10 11 mol cm −3 is comparable to that of other methods, including intracavity laser absorption spectroscopy.

Sub-40 μA continuous-wave lasing in an oxidized vertical-cavity surface-emitting laser with dielectric mirrors

Due to two improvements in cavity design, low-threshold lasing is achieved in oxidized vertical-cavity surface-emitting lasers incorporating upper dielectric distributed Bragg reflectors. Intracavity absorption is reduced by removal of a heavily p-type contact layer and the use of a low-loss MgF-ZnSe upper mirror. We report sub-100 μA lasing for a 7-μm diameter device, and sub-40 μA lasing for a 3-μm diameter device. The low-loss cavity design also allows for highly multimode operation at a low-bias current of 600 μA in a detuned cavity.

Loss of spectral memory in the relaxation oscillations of a multimode solid-state laser.

To test the application of solid-state lasers to intracavity laser absorption spectroscopy (ICLAS), we studied a neodymium-doped glass laser, in the presence of extreme relaxation oscillations (up to ${10}^{8}$ intensity contrast). The contrast of these oscillations could be varied by changing the Q-switch transition time. The recorded absorption spectra accurately follow the regular ICLAS behavior except when the intensity in the first relaxation oscillation minimum becomes too low. In such a case, we find that the ICLAS absorption spectrum accumulated during the first spike is ``forgotten'' by the system. Numerical simulations were done which accurately describe the observed transition to a regime of memory loss, when the number of photons circulating in each mode drops close to 1. These simulations show that the mechanism of memory loss is the prevailing of spontaneous over stimulated emission. This effect sets the fundamental limit of the sensitivity of an ICLAS spectrometer where the nonlinear mode interactions are negligible. This effect was observed before only in a diode laser system with high internal losses. In this paper we report an observation of ICLAS sensitivity saturation by spontaneous emission during relaxation oscillations in a laser with low cavity losses. \textcopyright{} 1996 The American Physical Society.

Simulation of the output characteristics and propagation of radiation from a CO laser with a selection cell

An investigation was made of the influence of an intracavity absorption cell containing water vapour on the spectral and energy characteristics of an electron-beam-controlled discharge-pumped CO laser. The absorption coefficients of laser radiation travelling along a model path containing dried air were calculated. Perturbations of the optical beam along the path was considered under conditions of forced transverse motion of air in the case of unmodified radiation and of radiation with a selected spectrum.

The analytical potential of diode laser-based intracavity absorption detection in column liquid chromatography

The possibility of on-line diode laser-based intracavity absorption detection (LIAD) in liquid chromatography is examined. Diode laser-based LIAD should, in principle, be very sensitive because of the inherent stability of the light source compared to that of other lasers. Because many diode lasers are sold in packages that also contain a photodiode for optical feedback stabilization, the detection set-up is very simple. However, contrary to earlier reports, little signal gain is observed for LIAD compared to direct absorption detection. This is mainly due to the inefficient coupling of the diode laser and the external cavity. Because LIAD is more sensitive to mechanical instabilities, while rationing with a reference signal as well as feedback stabilization is not possible, the actual detection limit achieved for the model compound mitoxantrone is 7 × 10 −7 M or 1 × 10 −4 absorption units. These values are about two orders of magnitude less favourable than those observed with double-beam absorption detection using the same diode lasers.

Spectrochemical analysis of trace contaminants in helium (helium–fluorine) pulsed discharge plasmas

Time-resolved spontaneous emission and absorption spectra in the 115–860 nm spectral range were studied for high pressure He and He–F2 pulsed discharge plasmas, the aim being the spectroscopic identification and on-line monitoring of transient and stable gaseous contaminants accumulated in an actual F2 laser gas mixture. Various radicals (CN, CH, C2, OH, SiF and CF2), excited atoms (O, C, Si, N and H), molecules (CO, O2 and N2) and molecular ions (N2+ and CO+) were identified as contaminants. The accumulation and self-transformation kinetics due to plasmo-chemical reactions between these species were investigated. It is suggested that carbon, oxygen and carbon monoxide species are desorbed as primary impurities from the metal electrodes during the sputtering of the electrodes from cathode (anode) hot spots. Secondary impurities such as transient radicals and more complex compounds are formed by plasmo-chemical reactions between primary and other trace impurities (H2, N2 and O2) and fluorine. These reactions might be strongly catalytically activated on the surface of pure nickel particles, sputtered from the electrodes. The sensitivity of the spontaneous emission methods was estimated to be better than 1 ppm for the detection of stable CO and N2 impurities in the He discharge plasma. Under static conditions, N2 and O2 impurities accumulate at a rate of about 0.1 mbar d–1 in He and He–F2 gas mixtures due to continuous outgassing of the laser chamber materials. Under F2 laser running conditions, a marked increase in the rate of accumulation of O2 contaminants (≈ 0.1 mbar per 5 × 105 shots) was observed. Oxygen is the main contaminant responsible for F2 laser output energy degradation because of the intracavity absorption at 157.6 nm in the oxygen dissociation absorption band. The strong absorption in the far VUV range (130–115 nm) observed in an aged He–F2 gas mixture was attributed to stable fluorine-containing compounds (tentatively identified as COF2 or HF molecules). In addition to the well known molecular fluorine emission band at 157 nm caused by the transition from the ionically bonded D′3Π2g state, two spontaneous emission bands (structural at 255 nm and continuous at 280 nm) were observed in the discharge of an He–F2 gas mixture. These new bands were attributed to transitions between covalently bonded excited electronic states of the fluorine molecule.

The rate constant for the recombination of trifluoromethyl radicals at T = 296 K

The high-pressure rate constant of the CF3 + CF3 → C2F6 reaction at T = 296 K was measured in the pulse photolysis (λ = 694.3 nm, ruby laser) of CF3NO in the presence of NO by means of the time-resolved detection of CF3NO by the intracavity absorption of He(SINGLE BOND)Ne laser radiation (λ = 632.8 nm). The obtained value is k2∞ = (3.9 ± 1.3) × 10−12 cm3/s. © 1996 John Wiley & Sons, Inc.

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 ?m

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS2. IR frequency tuning between 3076 and 3183 cm−1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)−1/2.

Distortion of self-induced-transparency solitons as a result of self-phase modulation in ion-doped fibers

The temporal envelope profile and the phase of a steady-state pulse propagating through a resonant medium in the presence of nonresonant nonlinearity are derived. The formation of solitonlike pulses takes place as a result of the balance of the self-phase modulation generated by nonresonant nonlinearity and the nonlinear resonant group-velocity dispersion induced by the self-induced-transparency effect in a resonant medium. Self-phase-modulation action leads to distortion of the pulse when its power and inverse duration exceed the critical values Pcr and τcr-1. We show the destructive role of self-phase modulation in the case of self-induced-transparency pulse generation in a laser with erbium-doped fiber as an intracavity coherent absorber.

Quasi-critical coupling between spatially resolved laser eigenstates: a novel approach to the measurement of intracavity absorption

A novel method of intracavity absorption measurement is demonstrated that is linear in absorber concentration. Strong competition between spatially resolved laser eigenstates is theoretically and experimentally shown to provide conveniently controlled and highly efficient optical leverage for intracavity sensing. In a first experimental realization, a He–Ne laser at 3.39 μm demonstrates a sensitivity of 3 × 109 molecules of methane in vacuum in the volume of the beam in the absorption cell, which corresponds to 0.6 part per billion times meters of CH4 in 1 atm of air. Good agreement is obtained between the experimental results and the theoretical predictions from a model of the laser dynamics based on Landau potentials associated with Lamb’s equations.

Measurement and Calculation of SiH2 Radical Density in SiH4 and Si2H6 Plasma for the Deposition of Hydrogenated Amorphous Silicon Thin Films

SiH2 radical density in SiH4 and Si2H6 plasmas diluted with He, Ar and Xe has been measured using intracavity laser absorption spectroscopy, and analyzed by a gas-phase reaction simulation. The density of SiH2 increases with dilution due to increase in dissociation rate of parent-gas molecules. The increase in the dissociation rate for He and Xe dilution originates in the increase of electron energy and density, respectively, and both of these contribute for Ar dilution.

Intracavity laser spectroscopy with vibronic solid-state lasers: II Influence of the nonlinear mode coupling on the maximum sensitivity of a Ti:sapphire laser

The time dynamics of the intracavity absorption in a Ti:sapphire intracavity laser spectrometer was studied in the long-generation time domain. Weak nonlinearity of the dependence of sensitivity on the generation time was observed for a standing-wave laser at 1 ms. Several possible mechanisms for nonlinear mode coupling that could be responsible for this effect are considered. Parasitic interference made the measurements for longer generation times difficult. In a ring configuration of the laser the parasitic interference was excluded. No nonlinearity was observed for atmospheric-water absorption up to an equivalent absorption path length of 900 km. An analysis of the spectral narrowing made it possible to conclude that the equivalent absorption path in the ring Ti:sapphire can exceed 30,000 km. Limitations that result from the birefringence of the laser crystal and ways of avoiding them are also discussed.

Intracavity absorption spectroscopy of the third stretching overtone transition of jet cooled methane

The third stretching overtone transition of jet cooled methane has been recorded between 11 170 and 11 315 cm−1 by intracavity laser absorption spectroscopy associated with a supersonic slit jet expansion. By varying the methane percentage seeded in the carrier gas, we have studied the evolution of the spectrum vs the rotational temperature. The rotational cooling drastically reduces the congestion of the spectrum and only a few lines are observed in the spectrum corresponding to the lowest rotational temperature. In the 11 245–11 315 cm−1 range, the rotational structure of two superimposed bands is clearly observed. The lowest rotational levels of each band are assigned and the corresponding band origins, rotational constants and Coriolis parameters are obtained. The two bands are assigned to the two components of a Fermi dyad corresponding to the interaction of the 3ν1+ν3 (F2) stretching level carrying the oscillator strength with a near resonant vibrational state of the background. The high mixing of the vibrational wave functions is reflected by the close values of the rotational constants obtained for the two bands. In the lower energy range of the spectrum (11 170–11 245 cm−1), two weaker vibrational bands are tentatively assigned.

Intracavity laser spectroscopy with vibronicsolid-state lasers I Spectrotemporal transient behavior of a Ti:sapphire laser

Spectrotemporal dynamics and the sensitivity limit of intracavity absorption in the case of a Ti: sapphire laser during buildup and relaxation oscillations are investigated. Both theoretical and experimental approaches confirm that the generation time, a crucial parameter in an intracavity absorption experiment, must be modified. During the transient period relaxation oscillations appear to have no effect on the regular linear growth of the absorption. The birefringence limits the sensitivity of the detection to an absorption coefficient of the order of 10−8 cm−1. A special cut of the crystal is suggested to overcome this technical limitation. It is shown that the ultimate limitation, imposed by spontaneous emission, promises a very high sensitivity, equivalent to an absorption path length of several hundred thousand kilometers. An example of the absorption of the 42 stretch band of CHD3 is presented.

Intensities and Self-Broadening Coefficients of Weak Water Vapor Lines in the 720-nm Region Determined by Intracavity Laser Absorption Spectroscopy

The time-resolved quasi-cw form of intracavity laser spectroscopy is used for the quantitative determination of absolute intensities and self-broadening coefficients of weak vibration-rotation lines of water that absorb in the region of the 720-nm band. The studies have been conducted using a 0.68-m-long intracavity absorption cell, and the data are analyzed both by directly fitting the absorption lineshape and by the curve of growth method. Values determined for some of the stronger lines are compared with the ones obtained in previous studies where multipass absorption cells and alternative instrumental techniques were employed. The comparisons show that the accuracy of the intracavity laser spectroscopy technique is comparable to that obtained by the use of other techniques. In addition, intensities and pressure-broadening parameters of several previously unanalyzed transitions are reported.

The quantitative determination of the S0—T1 absorption in pyrene with the use of intracavity absorption spectroscopy

Abstract The spin-forbidden S 0 — T 1 transition of pyrene was quantitatively investigated by applying intracavity absorption spectroscopy. The measurements were carried out in benzolic solutions. The molar decadic extinction coefficient of the 0—0 transition was determined to 6.0 × 10 −4 dm 3 mol −1 cm −1 . The integral absorptivity was calculated to 2.94 × 10 −2 dm 3 mol −1 cm −1 and yielded an intrinsic phosphorescence lifetime of 55.4 s. The intracavity absorptions were measured in a cavity-dumped Rhodamine 6G dye laser. Absorbances were recorded in the range between 8 × 10 −4 and 5 × 10 −6 .

Temporal Intracavity Absorbance Determination Using Pulsed Ti: Sapphire Laser

Abstract A tunable pulsed Ti: Sapphire laser was used to determine low concentrations using intracavity absorption spectroscopy with temporal resolution. The dependence of the laser-pulse evolution time on the absorbance of a sample introduced in the cavity (solution of 3,3-diethylthiatricarbocyanine iodide in dimethylsulphoxide) was studied theoretically and experimentally. The minimal experimentally measured absorbance was 8×10−4. The possibility was shown of using a Ti:S laser to detect absorbance of 10−5 via a temporal technique.