Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

High-resolution spectroscopy of SO2 using a frequency-doubled,continuous-wave dye laser

The SO₂ molecule is of considerable interest in the context of atmospheric pollution, and in many laser monitoring techniques the ultraviolet absorption band at 300 nm is used to determine SO₂ concentrations in the atmosphere. Recent laboratory experiments with a resolution of 2 × 10^-3 nm showed that variations could occur in absorption cross-section measurements made with different laser bandwidths due to unresolved fine structure. We have investigated absorption spectra with a line width of 3 × 10^-6 nm, using a frequency-doubled continuous-wave dye laser, and have confirmed the existence of fine structure in the absorption even when collisionally broadened with an atmosphere of nitrogen. These measurements provide a data base from which valid absorption cross sections may be calculated for all monitoring laser bandwidths. We estimate the pressure broadening coefficient for nitrogen in this wavelength region as 83 ± 38 kHz Pa^-1 (11 ± 5 MHz torr^-1). The temperature dependence of the absorption cross-section was also investigated.     

IR absorption of highly vibrationally excited SF6

In a double-resonance experiment, the absorption of various CO₂ laser lines by sulfur hexafluoride was measured, before and after the SF₆ was pumped by a fixed frequency CO₂ laser to a level of 5 quanta/molecule. The absorption is substantially shifted to longer wavelengths. But the short wavelength wing of the absorption band is not completely bleached. Instead a shoulder of several cm⁻¹ width is left. This shoulder is probably important for the explanation of the infrared laser induced dissociation of SF₆.     

Picosecond Z-scan study of bound electronic Kerr effect in LiNbO3 crystal associated with two-photon absorption

3 crystal have been measured using Z-scan technique with picosecond pulses at 532 nm. The nonlinear absorption coefficient and nonlinear refractive index are determined to be 2.5×10^-10 cm/W and 5.3×10^-15 cm²/W, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the reported Z-scan results in LiNbO₃ obtained with cw laser beam at 514 nm. The nonlinearities in LiNbO₃ induced by 532 nm picosecond pulses are believed to be mainly due to two-photon absorption and bound electronic Kerr effect associated with the two-photon absorption. Received: 4 July 1996     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

Silver nanoparticles produced by PLD in vacuum: role of the laser wavelength used

In this work we report the results of investigation of silver (Ag) nanoparticles prepared on a silica substrate by laser ablation. Our attention was focused on the mean diameter, size distribution and optical absorption properties of nanoparticles prepared in vacuum by using different laser wavelengths. The fundamental wavelength and the second, third, and fourth harmonics of a nanosecond Nd:YAG laser were used for nanoparticles fabrication. The corresponding values of the laser fluence for each wavelength were: 0.6 J/cm² at 266 nm, 0.8 J/cm² at 355 nm, 2.8 J/cm² at 532 nm, and 2 J/cm² at 1064 nm. The Ag nanoparticles produced have mean diameters in the range from 2 nm to 12 nm as the nanoparticles’ size decreases with the decrease of the wavelength used. The presence of the Ag nanoparticles was also evidenced by the appearance of a strong optical absorption band in the measured UV-VIS spectra associated with surface plasmon resonance (SPR). A redshift and widening of the absorption peak were observed as the laser wavelength was increased. Some additional investigations were performed in order to clarify the structure of the Ag nanoparticles.     

Absorption of CO2 laser pulses at different wavelengths by ground-state and vibrationally heated SF6

The absorption of CO₂ laser pulses by low pressure SF₆ gas has been investigated over a wide range of energy fluxes. For laser energy fluxes of 0.01–1 J cm^-2 the effective absorption cross section varies between 0.2 and 2 × 10^-18 cm². For each laser line an individual dependence on the energy is found and in some cases minor changes in the absorption behaviour seem to occur around 0.1 J cm^-2. SF₆ excited with an average vibrational energy content of up to 20 photons/molecule does not absorb measurable amounts of 9.4 μm laser light. The influence of various SF₆ and Ar pressures on the temporal shape of the transmitted pulses has been investigated.     

CF3CDCl2分子红外多光子光热吸收谱

本文报道用铂丝光热吸收池作为探测器得到了CF₃CDCI₂的红外多光子吸收谱。并发现在线性吸收谱中944cm^-1处的吸收峰在多光子吸收谱中分裂为947cm^-1和927cm^-1两个吸收峰。这一现象与科里奥利力和非谐性引起的简正振动的耦合作用有关。这样,在用CF₃CDCI₂/CF₃CHCI₂体系分离氘同位素时,可以根据这一新结果选择更合适的激发波长。实验还发现在线性吸收谱中986cm^-1处的吸收峰在多光子吸收谱中出现约6cm^-2的红移。这种红移现象起源于分子振动能级的非谐性。 关键词：     

Pulsed broadly tunable room-temperature Cr2+:CdS laser

We report the spectroscopic characteristics and laser properties of Cr²⁺:CdS. The emission lifetime of the exited state, ⁵E, of Cr²⁺ in CdS host was measured to be 0.93 μs at room temperature and 7.3 μs at liquid nitrogen temperature. The measured values of peak absorption and emission cross sections were found to be 1.4×10⁻¹⁸ and 1.1×10⁻¹⁸ cm² respectively. Room-temperature pulsed-laser operation was obtained under pumping by the 1.94 μm output of a Tm:YAP laser. The output of the free-running Cr²⁺:CdS laser was centered at ∼2.6 μm, and an absorbed energy slope efficiency of 38.8% was demonstrated. The passive crystal losses were measured to be less than 0.049 cm⁻¹ at the lasing wavelength. With an intracavity prism, the Cr²⁺:CdS laser was tuned from 2.18 to 3.32 μm.     

Side-pumped continuous-wave Cr:Nd:YAG ceramic solar laser

To clarify the advantages of Cr:Nd:YAG ceramics rods in solar-pumped lasers, a fused silica light guide with rectangular cross-section is coupled to a compound V-shaped cavity within which a 7 mm diameter 0.1 at.% Cr:1.0 at.% Nd:YAG ceramic rod is uniformly pumped. The highly concentrated solar radiation at the focal spot of a 2 m diameter stationary parabolic mirror is transformed into a uniform pump radiation by the light guide. Efficient pump light absorption is achieved by pumping uniformly the ceramic rod within the V-shaped cavity. Optimum pumping parameters and solar laser output powers are found through ZEMAX^© non-sequential ray-tracing and LASCAD^© laser cavity analysis codes. 33.6 W continuous-wave laser power is measured, corresponding to 1.32 times enhancement over our previous results with a 4 mm diameter Nd:YAG single-crystal rod. High slope efficiency of 2.6 % is also registered. The solar laser output performances of both the ceramic and the single-crystal rods are finally compared, revealing the relative advantage of the Cr:Nd:YAG rod in conversion efficiency. Low scattering coefficient of 0.0018 cm^?1 is deduced for the ceramic rod. Heat load is considered as a key factor affecting the ceramic laser output performance.     

High-peak-power,high-repetition-rate LD end-pumped Nd:YVO<Subscript>4</Subscript> burst mode laser

A compact high-peak-power, high-repetition-rate burst mode laser is achieved by an acousto-optical Q-switched Nd:YVO₄ 1064 nm laser directly pumped at 878.6 nm. Pulse trains with 10–100 pulses are obtained using acousto-optical Q-switch at repetition rates of 10–100 kHz under a pulsed pumping with a 1 ms duration. At the maximum pump energy of 108.5 mJ, the pulse energy of 10 kHz burst mode laser reaches 44 mJ corresponding to a single pulse energy of 4.4 mJ and an optical-to-optical efficiency of 40.5 %.The maximum peak power of ~468.1 kW at 10 kHz is obtained with a pulse width of 9.4 ns. The beam quality factor is measured to be M ² ~1.5 and the pulse jitter is estimated to be less than 1 % in both amplitude and time region.     

A coherent light source,widely tunable down to 16 μm by stimulated Raman scattering

An infra-red emission tunable down to 16 μm is produced by a hydrogen Stokes shifted dye laser. The peak power measured at 16 μm is of ca. 50 kW in 2 ns within a one millimeter diameter beam. The corresponding light flux of the order of 5 MW cm^-2 could allow a multi-step vibrational excitation of molecules absorbing down to 16 μm. Simultaneous emissions from the laser and from the previous Stokes bands are available at: 0.76, 1.62, 2.09 μm with respective peak powers of 550, 50 and 20 MW. Spectral widths of 0.8 cm^-1 and of ca. 7 × 10^-2 cm^-1 are obtained for the dye laser and for each of the Stokes band respectively.     

A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

Analysis of low power spatial light modulation characteristics of bacteriorhodopsin

We present a detailed and accurate analysis of low power spatial light modulation characteristics of bacteriorhodopsin (bR) based on nonlinear intensity induced absorption. Amplitude modulation of probe laser read beam transmissions at 410 nm and 640 nm, corresponding to the peak absorption of M^II and O states of D96N bR and WT bR respectively, by the modulation laser write beam intensity-induced population changes at 570 nm has been analyzed, considering all intermediate states with both forward and backward transitions in the respective bR photocycles, using the rate equation approach. The SLM characteristics are shown to be sensitive to the normalized small signal absorption coefficient β, rate constants of M^II and O intermediate states and the absorption cross-section of the initial B state at the probe wavelength (σ_Bp). There exists an optimum value of β for which maximum percentage modulation can be achieved. It is shown that for extended M^II state lifetime of 250 s in D96N bR and O state lifetime of 2.2 s in WT bR, with σ_Bp= 0, 100% modulation of read beam transmissions can be achieved, leading to high dynamic range and sensitivity for low laser write beam intensities of 50 μW/cm² and 4 mW/cm² at 570 nm, respectively.     

Generation of narrowband tunable VUV radiation at the Lyman-α wavelength

Tunable narrowband VUV radiation has been generated at the Lyman-α wavelength $\text{λ =}\text{1216}\text{\&}\text{A}\text{ring}$; by frequency tripling in krypton the frequency-doubled output of a powerful dye laser system which is excited with the second harmonic of a Nd-YAG laser. 5 ns long UV dye laser pulses ($\text{λ =}\text{3646}\text{A}\text{?}$) of 1.8 MW peak power yielded VUV light pulses of 2.2 W (5.4 × 19⁹ photons/pulse). The bandwidth of the dye laser radiation could be narrowed to 8.7 × 10^-3cm^-1 (4.6 × 10^-3 Å). The expected bandwidth of the VUV is less than 5.2 × 10^-2cm^-1 (7 × 10^-4 Å). The tunable VUV radiation is used for the recording of the absorption spectra of the Lyman-α resonance transitions in atomic hydrogen and deuterium with doppler-limited resolution.     

Nonlinear absorption and optical damage threshold of carbon-based nanostructured material embedded in a protein

Physical processes in laser–matter interaction used to be determined by generation of fast electrons resulting from efficient conversion of the absorbed laser radiation. Composite materials offer the possibility to control the absorption by choice of the host material and dopants. Reported here strong absorption of ultrashort laser pulse in a composite carbon-based nanomaterial including single-walled carbon nanotubes (SWCNTs) or multilayer graphene was measured in the intensity range between 10¹² and 10¹⁶ W cm^?2. A protein (lysozyme) was used as the host. The maximum absorption of femtosecond laser pulse has reached 92–96 %. The optical damage thresholds of the coatings were registered at an intensity of (1.1 ± 0.5) × 10¹³ W cm^?2 for the embedded SWCNTs and at (3.4 ± 0.3) × 10¹³ W cm^?2 for the embedded graphene. Encapsulated variant of the dispersed nanomaterial was investigated as well. It was found that supernatant protein in the coating material tends to dominate the absorption process, independently of the embedded nanomaterial. The opposite was observed for the encapsulated material.     

Nonlinear optical characteristics of a pseudoisocyanine solution

The saturated nonlinear absorption and Kerr nonlinearities of an aqueous pseudoisocyanine solution are investigated at the wavelengths of 532 and 529 nm with the use of pulsed laser radiation of different duration (8 ns and 475 fs). The measured values of the nonlinear refractive indices amount to ?6×10^?12 (t=8 ns) and ?8×10^?14 cm²W^?1 (t=475 fs). The change in self-action effects in pseudoisocyanine from self-defocusing to self-focusing revealed in the case of increasing intensity of femtosecond laser pulses is attributed to the effect of a fifth-order nonlinear optical process. The nonlinear refractive index responsible for this process amounts to 4×10^?24 cm⁴ W^?2. The imaginary part of the third-order nonlinear susceptibility, responsible for the induced transparency of the pseudoisocyanine solution, is Imχ ⁽³⁾=?2×10^?12 esu. Temporal changes in the shape of nanosecond laser pulses due to the nonlinear refraction induced by a thermal process are analyzed.     

Absorption spectrum of H<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> near 1.06 μm

The absorption spectra of molecular hydrogen plasma excited by electric hollow-cathode and high-frequency discharges are measured. The spectra in the region of 1.06 μm were recorded using a neodymium intracavity laser spectrometer with a resolution of 0.03 cm^?1 and an absorption sensitivity of 10^?8 cm^?1. The absorption lines that can be attributed to the transitions to vibrational states in the molecule are recorded.     

The far-infrared absorption spectrum of electron-hole drops in silicon

We report on the first observation of the absorption spectrum of electron-hole drops in silicon. The absorption at 14°K consists of a broad peak at 34.2 ± 0.2 meV in the far-infrared spectral range. The lineshape can be well fit with a model that uses Mie theory of light absorption by small particles. The model includes both intra and interband terms. We find a plasma frequency of 51.9 ± 0.4 meV and from this we calculate a electron-hole density in the drops of (3.37 ± 0.06) × 10¹⁸ cm^?3.     

Optically pumped 15.90 μm SF6 laser

Optically pumped vibrational transition lasing has been achieved for the first time in a nonlinear molecule. Laser radiation at 628.74 ± 0.02 cm^?1 was generated from SF₆ using CO₂ TEA laser excitation. The SF₆ pumping is shown to be via absorption of two photons.