Lunhua Deng

Ultrabroadband tunable continuous-wave difference-frequency generation in periodically poled lithium niobate waveguides

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.

Perturbation analysis of the υ = 6 level in the d3Δ state of CS based on its near-infrared absorption spectrum.

The spectrum of CS was recorded in the region of 12,086-12,630 cm(-1) by employing optical heterodyne concentration modulation laser absorption spectroscopy. Nearly 350 transitions were assigned to the (6, 0) band in the d(3)Δ-a(3)Π system of CS. The overtone transitions of the (12, 0) band in the a(3)Π(2)-a(3)Π(0) transition were first observed due to the perturbation interaction between d(3)Δ(1) and a(3)Π(2). The Λ doubling in the a(3)Π(1) state was also resolved at high rotational levels. The molecular constants of the a(3)Π (υ = 0) and d(3)Δ (υ = 6) levels and the perturbation parameters of the d(3)Δ (υ = 6) level were determined through nonlinear least-squares fitting using effective hamiltonians. The calculations of mixing fractions of the perturbed states were performed in order to obtain precise information on the perturbations of the d(3)Δ (υ = 6) levels. The mechanisms for perturbations of d(3)Δ (υ = 6) with the a(3)Π (υ = 12) and A(1)Π (υ = 1) levels, especially for the second-order perturbation, were discussed and explained according to first-order nondegenerate perturbation theory.

Continuous-wave difference-frequency generation in periodically poled RbTiOAsO 4 crystal and application to isotopic analysis of methane

We report on development of a widely tunable infrared (3.4-4.5 mum) continuous-wave laser spectrometer based on difference frequency generation in a periodically poled RbTiOAsO₄ crystal and its application to isotope analysis of ¹³CH₄/¹²CH₄.

Absorption Spectroscopy of (8,1) Band in d3Δ-a3Π System of CS Radical

The CS radical was generated by discharging the mixture gas of CS2 and Helium. The Doppler limited spectra of CS were recorded in the region of 12350–12950 cm−1 using optical heterodyne concentration modulation absorption spectroscopy. Three hundred and twenty-six lines were recorded and assigned to the d3Δ-a3Π (8,1) band, in which eighty-three transitions were first observed. A set of improved molecular constants for the d3Δ(v=8) and a3Π(v=1) levels were determined by a non-linear least-squares fitting of all the lines to the effective Hamiltonian.

Portable remote sensor of methane leakage using near-IR diode laser

A portable sensor based on tunable diode laser for remote sensing of methane (CH4) leakage at a wavelength of 1.65 μm was developed. Wavelength modulated spectroscopy technology is also applied. It was found that the sensor can detect a 10 cm3 min-1 city-gas leak with a sensor output equivalent to the range-integrated concentration over 100 ppm-mn.

High-speed multi-pass tunable diode laser absorption spectrometer based on frequency-modulation spectroscopy

We report a multi-pass tunable diode laser absorption spectrometer based on the frequency-modulation spectroscopy (FMS) technique. It has the advantage of high scan speed and is immune to the etalon effect. A multi-pass Herriott-type cell was used in the spectrometer to increase the effective optical length to 17.5 m and compact the physical dimensions of the spectrometer to 60×30×30 cm3. Noise due to low-frequency fluctuation of the laser power and the 1/f noise in the rapid detection are sufficiently reduced by FMS. Interference fringes are effectively suppressed when the modulation frequency equals to integer or half-integer times of their free spectral range (FSR). An absorption line of C2H2 around 1.51 µm was recorded with the spectrometer to demonstrate its capabilities. The response frequency of the spectrometer is up to 100 kHz (10 µs) thanks to the high modulation frequency of FMS. The detection sensitivity of the spectrometer is about 240 ppb (3σ) at 100 kHz measurement repetition rate. The amplitude of the absorption signal is highly linear to the C2H2 concentration in the range of 300 ppb -100 ppm. Based on the Allan variation, the detection limit was determined to be 18 ppb with a detection time of 166 s.

Pressure-Dependent Detection of Carbon Monoxide Employing Wavelength Modulation Spectroscopy Using a Herriott-Type Cell

Wavelength modulation spectroscopy (WMS) combined with a multipass absorption cell has been used to measure a weak absorption line of carbon monoxide (CO) at 1.578 µm. A 0.95m Herriott-type cell provides an effective absorption path length of 55.1 m. The WMS signals from the first and second harmonic output of a lock-in amplifier (WMS-1f and 2f, respectively) agree with the Beer–Lambert law, especially at low concentrations. After boxcar averaging, the minimum detection limit achieved is 4.3 ppm for a measurement time of 0.125 s. The corresponding normalized detection limit is 84 ppm m Hz–1/2. If the integrated time is increased to 88 s, the minimum detectable limit of CO can reach to 0.29 ppm based on an Allan variation analysis. The pressure-dependent relationship is validated after accounting for the pressure factor in data processing. Finally, a linear correlation between the WMS-2f amplitudes and gas concentrations is obtained at concentration ratios less than 15.5%, and the accuracy is better than 92% at total pressure less than 62.7 Torr.

Rotational Analysis of A2Πu-X2Πg System of 16O2+ Cation

The Doppler-limited absorption spectrum of 16O2+ cation was observed in the region of 11385–12100 cm−1 by optical heterodyne velocity modulation absorption spectroscopy (OH-VMS). The transitions were assigned to the (2, 19), (3, 20), and (5, 21) bands in the second negative system (A2Πu-X2Πg). All the available lines measured using OH-VMS were global fitted in a nonlinear least-squares fitting procedure, and precise molecular constants (Bv, Av, Dv, pv, qv, γv) were obtained for the involved levels.

Predissociation of the b3Πg (v = 9) State of He2 Excimer

The predissociation of the v = 9 level in the b3Πg state by the c3Σg+ state of helium eximer (He2) was studied based on the newly observed (9, 3) band in the b3Πg−a3Σu+ system in the region of 12065–12445 cm−1 employing optical heterodyne-concentration modulation absorption spectroscopy. With the help of the previous potential energy curves and molecular constants of He2, the corresponding predissociation mechanism for the b3Πg (v = 9) state was analyzed. An RKR potential energy curve of b3Πg and an ab initio potential curve of c3Σg+ were used to calculate the predissociation linewidths that show basic agreement with observations, which can quantitatively explain the experiments.

Absorption spectrum of neutral phosphorus (P I) in the range 7700–8256 Å

The absorption spectrum of neutral phosphorus was recorded in the range 7700–8256 A using optical heterodyne magnetic rotation concentration modulation spectroscopy with a tunable Ti:sapphire laser as the light source. Phosphorus atoms were excited by discharging the mixed gases of phosphorus vapor and neon gas. The phosphorus vapor was prepared by heating white phosphorus chips placed on a cylindrical copper electrode during discharge. In total, 234 lines of P I were observed, of which 127 were assigned. The transition frequencies were obtained with an uncertainty of 0.007 cm−1. Empirical line intensities were provided covering four orders of magnitude.