Time-resolved Fourier transform intracavity spectroscopy with a Cr2+:ZnSe laser

Intracavity absorption spectroscopy of HCl isotopes, H2O, CH4, C2H4, and C2H6 in the 3.1-3.4 µm spectral range using a Cr:CdSe laser

We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around λ = 3.1-3.4 µm. A detection scheme based on an integrated recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of L_eff ≈ 850 m. Exemplary measurements of atmospheric H₂O and CH₄, and additionally introduced gas-phase HCl, C₂H₄, or C₂H₆ are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 10⁴ are discussed.

Room-temperature Fe:ZnSe laser tunable in the spectral range of 3.7-5.3 µm applied for intracavity absorption spectroscopy of CO2 isotopes, CO and N2O

We demonstrate an intracavity absorption spectroscopy system based on a broadband single-crystal pulsed Fe:ZnSe laser. The laser operates at room-temperature and is continuously tunable in the spectral range of 3.76-5.29 µm. The long-wavelength emission up to 5.29 µm is a record achievement for Fe:ZnSe lasers, to the best of our knowledge. The developed laser system is applied for measurements of gaseous absorption inside the laser resonator. We demonstrate sensitive detection of (i) CO₂ isotopes in the atmosphere and in human breath, (ii) CO in breath (after cigarette smoking) and in the smoke of a smoldering paper, and (iii) N₂O in a gas flow. The achieved detection limits are: 0.1 ppm for ¹²CO₂ and ¹³CO₂, 3 ppm for CO, and 1 ppm for N₂O. The sensitivity of the current system is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 10⁷ are discussed.

All-fiber nonlinear optical wavelength conversion system from the C-band to the mid-infrared

We demonstrate an all-fiber wavelength conversion system from the C-band to the wavelength range of 2.30-2.64 µm of the mid-infrared (MIR). A series of nonlinear processes is used to perform this spectral shift in excess of 80 THz; from optical pulses in the C-band, self-phase modulation spectral broadening and offset filtering generate probe pulses in the C- and L-band. In parallel to this, Raman-induced soliton self-frequency shift converts pulses from the C-band into pump pulses in the 2 µm wavelength band. The resulting synchronized probe and pump pulses interact via degenerate four-wave mixing to produce wavelength-converted idler pulses in the MIR. Silica fiber is used for nonlinear processes at wavelengths $ {\lt} 2\;{\unicode{x00B5}{\rm m}}$<2µm whereas chalcogenide glass is used for nonlinear processes at wavelengths $ {\ge} 2\;{\unicode{x00B5}{\rm m}}$≥2µm. This system is a major step toward the development of compact MIR optical sources generated from widespread pump lasers of the C-band.

High-energy thermoelectrically cooled Fe:ZnSe laser tunable over 3.75-4.82 μm

The characteristics of an Fe:ZnSe laser thermoelectrically cooled to 220 K are described. Output energy of 7.5 J and optical-to-optical efficiency of 30% have been demonstrated in single-shot operation at 4.3 μm with a 2.94 μm Er:YAG pump laser. By using an intracavity prism, continuous tuning from 3.75 to 4.82 μm has been obtained at output energy up to 3.1 J.

Continuous-wave broadly tunable high-power Cr:CdS laser

We report spectroscopic characteristics and laser properties of the mid-infrared active laser medium Cr²⁺:CdS. Temperature-dependent absorption, luminescence and lifetime measurements of the ⁵E exited state allow determination of peak emission cross section value of 1.35 × 10^-18 cm² in σ-polarization at room temperature. Lifetime values vary from 7.6 µs at 8 K to 0.48 µs at 320 K, corresponding to 22 % quantum yield at 285 K. Under Tm-fiber laser pumping, the continuous-wave output reached 1.8 W at 2.5 μm with 35.5 % slope efficiency. With a single CaF₂ prism, the CW Cr²⁺:CdS laser could be tuned from 2.240 to 3.285 µm.

High-resolution broadband spectroscopy with a resonator-based phase modulator

A method for significant enhancement of the spectral resolution of a Fabry-Perot resonator in transmission and absorption measurements is proposed. In the method, a laser with ultrashort pulses is used as the optical source. A dispersive element is placed in front of the Fabry-Perot resonator and a phase modulator is incorporated into the resonator. The spectrum of the laser pulse transmitted through the system is approximately periodic with ultranarrow peaks. The sample transmission spectrum is measured by scanning the output pulse spectrum. It is demonstrated, in numerical simulations, that for realistic parameters of the phase modulator, the finesse of the Fabry-Perot resonator is increased from 72 to 1900 and a resolution of 1 MHz is achieved. A method for increasing the spectral range of measurements with scanning the periodic spectra is also proposed. The method is based on the use of a waveguide array of Mach-Zehnder interferometers or a single discretely tunable interferometer. The measurement of the sample transmission spectrum within 33 free spectral ranges of the resonator is numerically demonstrated. The spectral range of the measurement can be increased up to 10 THz resulting in the equivalent finesse of the system of 10(7) for a 100 fs laser pulse.

Soliton absorption spectroscopy

We analyze optical soliton propagation in the presence of weak absorption lines with much narrower linewidths as compared to the soliton spectrum width using the novel perturbation analysis technique based on an integral representation in the spectral domain. The stable soliton acquires spectral modulation that follows the associated index of refraction of the absorber. The model can be applied to ordinary soliton propagation and to an absorber inside a passively modelocked laser. In the latter case, a comparison with water vapor absorption in a femtosecond Cr:ZnSe laser yields a very good agreement with experiment. Compared to the conventional absorption measurement in a cell of the same length, the signal is increased by an order of magnitude. The obtained analytical expressions allow further improving of the sensitivity and spectroscopic accuracy making the soliton absorption spectroscopy a promising novel measurement technique.

Sensitive multiplex spectroscopy in the molecular fingerprint 2.4 mum region with a Cr(2+):ZnSe femtosecond laser

An ultrashort-pulse Cr(2+):ZnSe laser is a novel broadband source for sensitive high resolution molecular spectroscopy. A 130-fs pulse allows covering of up to 380 cm(-1) spectral domain around 2.4 mum which is analyzed simultaneously with a 0.12 cm(-1) (3.6 GHz) resolution by a Fouriertransform spectrometer. Recorded in 13 s, from 70-cm length absorption around 4150 cm(-1), acetylene and ammonia spectra exhibit a 3800 signal-tonoise ratio and a 2.4.10(-7) cm(-1).Hz(-1/2) noise equivalent absorption coefficient at one second averaging per spectral element, suggesting a 0.2 ppbv detection level for HF molecule. With the widely practiced classical tungsten lamp source instead of the laser, identical spectra would have taken more than one hour.

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.

N(2)O weak lines observed between 3900 and 4050 cm from long path absorption spectra

Previously unobserved nitrous oxide transitions around 2.5 μm are measured by intracavity laser absorption spectroscopy (ICLAS) analyzed by time-resolved Fourier transform (TRFT) spectrometer. With an accuracy of the order of 10(-3) cm(-1), measured positions of 1637 assigned weak transitions are provided. They belong to 42 vibrational transitions, among which 33 are observed for the first time. These data are believed to be useful in particular to monitoring atmosphere purposes.

Acetylene weak bands at 2.5 μm from intracavity Cr:ZnSe laser absorption observed with time-resolved Fourier transform spectroscopy

The spectral dynamics of a mid-infrared multimode Cr(2+):ZnSe laser located in a vacuum sealed chamber containing acetylene at low pressure is analyzed by a stepping-mode high-resolution time-resolved Fourier transform interferometer. Doppler-limited absorption spectra of C(2)H(2) in natural isotopic abundance are recorded around 4000 cm(-1) with kilometric absorption path lengths and sensitivities better than 3 10(-8) cm(-1). Two cold bands are newly identified and assigned to the ν(1)+ν(4) (1) and ν(3)+ν(5) (1) transitions of (12)C(13)CH(2). The ν(1)+ν(5) (1) band of (12)C(2)HD and fourteen (12)C(2)H(2) bands are observed, among which for the first time ν(2)+2ν(4) (2)+ν(5) (-1).