Sheng Wu

A nanosecond optical parametric generator/amplifier seeded by an external cavity diode laser

We report on the development of an injection-seeded nanosecond optical parametric generator/amplifier (ns-OPG/OPA) that combines relatively low thresholds with good conversion efficiencies. Because there is no cavity whose optical properties must be actively controlled to match the injected radiation field, external seeding over wide ranges by CW single longitudinal mode (SLM) lasers is straightforward. The seeded ns-OPG/OPA spectral resolution (650±150 MHz) is close to the pump-limited bandwidth, and the output beam has a small angular divergence (∼0.5 mrad at 628 nm). Other properties of the seeded ns-OPG/OPA, such as the efficiency (>30%), the threshold for external seeding (∼1–3 mW), and the locking range (>20–30 cm^(−1)), are also characterized. The continuous scanning and narrow spectral bandwidth of the OPG/OPA is verified in photo-acoustic absorption experiments on the third O–H stretching overtone of water near 815 nm.

Simple, high-performance type II β-BaB2O4 optical parametric oscillator

A visible /near-IR optical parametric oscillator (OPO) based on type II phase matching in ?-BaB2 O4 (BBO) is described. Pumped at 355 nm, this OPO covers 410 -2500 nm completely with a single set of standard Nd:YAG cavity optics. The output efficiency is >25 %, the linewidth of the OPO is narrower than 1 -2 cm-1 without the use of gratings or etalons, and the signal-beam divergence is <400 ?rad. Three type I BBO doubling crystals are used to extend the tuning range from 208 to 415 nm. Doubling efficiencies as high as 40 % are easily obtained. The reasons for the high doubling and overall system efficiency are discussed.

A multicrystal harmonic generator that compensates for thermally induced phase mismatch

A multicrystal means of compensating for the thermally induced phase mismatch encountered in the generation of high average power UV radiation is described. The concept is experimentally tested with a Nd:YAG laser 4th harmonic generator based on two β-BaB_2O_4 (BBO) crystals. Single versus two crystal results demonstrate that this design compensates for the thermally induced phase mismatch, effectively increasing the interaction length of nonlinear optical crystals during harmonic generation under high loading.

Two-photon absorption inside β-BBO crystal during UV nonlinear optical conversion

We measured the two-photon absorption (TPA) cross sections inside (beta) -BBO crystal during UV harmonic generation. We found that the 2-photon absorption is dominating the absorption effect inside the BBO crystal during UV harmonic generation. Both 2 UV photons and 1 UV photon + 1 fundamental photon absorption cross sections are significant. Possible explanations are presented, and compared with other nonlinear otpical crystals. Thermal profiles inside the crystal as a result of the strong absorption processes are discussed through a computer program that simulates the heat dissipation process. We conclude that TPA is the significant factor in high power scaling of UV harmonic generation inside nonlinear optical crystals.

A field-deployable compound-specific isotope analyzer based on quantum cascade laser and hollow waveguide

A field deployable Compound Specific Isotope Analyzer (CSIA) coupled with capillary chromatogrpahy based on Quantum Cascade (QC) lasers and Hollow Waveguide (HWG) with precision and chemical resolution matching mature Mass Spectroscopy has been achieved in our laboratory. The system could realize 0.3 per mil accuracy for 12C/13C for a Gas Chromatography (GC) peak lasting as short as 5 seconds with carbon molar concentration in the GC peak less than 0.5%. Spectroscopic advantages of HWG when working with QC lasers, i.e. single mode transmission, noiseless measurement and small sample volume, are compared with traditional free space and multipass spectroscopy methods.

Quantum cascade laser sensors for online gas chromatography

We show that QC laser could improve capillary Gas Chromatography Infrared spectroscopy resolution significantly, i.e. both Doppler limited and Doppler free resolution could be achieved. To achieve these goals, we report our latest efforts in characterizing the tuning and noise properties of Quantum Cascade (QC) lasers; novel schemes on modulation to gain largest tuning range as well as on stabilizing and locking the QC lasers are proposed, and results presented.

Observation of whispering gallery modes in the mid-infrared with a quantum cascade laser: possible applications to nanoliter chemical sensing

Excitation of the whispering gallery modes (WGM) of a CaF2 ball resonator is demonstrated at 4.5 micron with a pulsed Quantum Cascade laser. A prism coupling scheme for mid-infrared is described. Future applications of WGM resonators as hyphenated inline chromatography sensors are discussed.

Dual Fabry–Perot filter for measurement of CO rotational spectra: design and application to the CO spectrum of Venus

We present the design of a harmonic resonant filter that can be used with a Fourier transform spectrometer (FTS) for simultaneous measurement of a series of lines in the CO rotational ladder. To enable studies of both broad CO absorptions in Venus and modestly red-shifted CO emission from external galaxies, relatively broad (approximately 10-30-GHz FWHM) transmission passbands are desirable. Because a single low-finesse Fabry-Perot (FP) etalon has insufficient interline rejection, a dual-FP etalon was considered. Such a design provides significantly better interband rejection and somewhat more flattopped transmission spikes. A prototype filter of this type, made of two thin silicon disks spaced by an air gap, has been constructed and used with our FTS at the Caltech Submillimeter Observatory for simultaneous measurement of the four submillimeter CO transitions in the atmosphere of Venus that are accessible from the ground.

A minimized ultra-sensitive MIR hollow waveguide (HWG) isotope ratio analyzer for environmental and industrial applications

An advanced commercial InfraRed Isotope Ratio (IR2) analyzer has been developed in Arrow Grand Technologies and the hollow waveguide (HWG) is used as the sample tube. By measuring the selected CO2 absorption peaks in the Mid- IR(MIR), the stable carbon isotope ratio, i.e. 13C, is obtained at a fast sampling rate. Combined with a GC and a combustor, it has been successfully employed to measure compound specific 13C isotope ratios in the field. By updating the single path HWG to 5-path HWG, we have also demonstrated its application for the environmental and health research. Here, the isotope ratio analyzer is minimized in size and weight to better fulfill the field deployment requirements. The size is reduced from 80cm*51cm*51cm to 74cm*33cm*46cm. After optimizing all subsystems, the minimized isotope ratio analyzer has a better performance. What’s more, the number of HWG paths can be selected to match the specific application. For instance, a 3-pass HWG is selected to conduct ultra-sensitive compound specific isotope analysis for mud gas logging, and a 5+1 pass HWG could measure isotope ratios of carbon with an ultra-broad CO2 concentration range of 300 ppm-47,500 ppm and a fast sample refresh and data processing rate up to 10Hz.

MIR hollow waveguide (HWG) isotope ratio analyzer for environmental applications

An advanced commercial Mid-InfraRed Isotope Ratio (IR2) analyzer was developed in Arrow Grand Technologies based on hollow waveguide (HWG) as the sample tube. The stable carbon isotope ratio, i.e. δ13C, was obtained by measuring the selected CO2 absorption peaks in the MIR. Combined with a GC and a combustor, it has been successfully employed to measure compound specific δ13C isotope ratios in the field. By using both the 1- pass HWG and 5-path HWG, we are able to measure δ13C isotope ratio at a broad CO2 concentration of 300 ppm-37,500 ppm. Here, we demonstrate its applications in environmental studies. The δ13C isotope ratio and concentration of CO2 exhaled by soil samples was measured in real time with the isotope analyzer. The concentration was found to change with the time. We also convert the Dissolved Inorganic Carbon (DIC) into CO2, and then measure the δ13C isotope ratio with an accuracy of better than 0.3 ‰ (1 σ) with a 6 min test time and 1 ml sample usage. Tap water, NaHCO3 solvent, coca, and even beer were tested. Lastly, the 13C isotope ratio of CO2 exhaled by human beings was obtained <10 seconds after simply blowing the exhaled CO2 into a tube with an accuracy of 0.5‰ (1 σ) without sample preconditioning. In summary, a commercial HWG isotope analyzer was demonstrated to be able to perform environmental and health studies with a high accuracy (~0.3 ‰/Hz1/2 1 σ), fast sampling rate (up to 10 Hz), low sample consumption (~1 ml), and broad CO2 concentration range (300 ppm-37,500 ppm).