Two-wavelength and power-balanced oscillation of a CO2 laser for application to differential absorption measurements

A new technique of alternate two-wavelength oscillation of a CO₂ laser is discussed for application to various differential absorption spectroscopic measurements. Power-balanced, two-adjacent branch oscillation using a single CO₂ laser was achieved by modulating the angle of a mirror inside the laser cavity and adjusting automatically the cavity length. The two-wavelength modulation frequency was extended up to about 1.2 kHz. Line tuning and power modulation characteristics were studied. The laser was used in long-path differential absorption measurements of ethylene air pollution molecules to demonstrate the capability of this power-balanced, two-wavelength oscillation method. The minimum detectable absorption was nearly 1×10^–3 in a short-path cell experiment and 3×10^–3 in a long-path experiment.     

Simultaneous tunable two-wavelength ultraviolet dye laser

A dye laser system allowing simultaneous tunable two-wavelength oscillationin the ultraviolet has been developed. An intra-cavity dual beam system employs one diffraction and grating frequencydoubling at both wavelengths is achieved with bidirectional illumination of a single KDP crystal. The system characteristics will be discussed with respect to application in a differential absorption lidar operating around 300 nm for atmospheric SO₂ detection.     

He-Ne laser pumped dimer lasers

Continuous laser oscillation of the dimer molecules Na₂, K₂ and I₂ on more than 40 lines in the spectral range of 685 nm to 1170 nm has been obtained by optical pumping with a He-Ne laser of up to 25 mW output power. Thresholds as low as 1 mW and output powers up to 1.5 mW could be achieved. Properties of these simple optical conversion system will be discussed.     

2-μm high-power multiple-frequency single-mode Q-switched Ho:YLF laser for DIAL application

We report on the development and the demonstration of a two-wavelength single-frequency laser oscillator based on Ho:YLF crystal. This laser is especially suitable for application as a transmitter in differential absorption lidar (DIAL)/integrated path differential absorption (IPDA) measurements of atmospheric carbon dioxide (CO₂) using the R30 CO₂ absorption line at 2,050.967 nm. The oscillator consists in a fiber-coupled and free-space solid-state hybrid system and can be used in high-energy middle-rate or moderate-energy high-rate configurations. The latter produced On and Off sequentially single-frequency laser pulses with 13 mJ of energy at a repetition rate of 2 kHz and pulse duration of 42 ns. The pulse energy and frequency stabilities are specially documented in free-running, single-frequency and two-frequency seeding single-mode operations. Standard deviation is 7.7 % for pulse energy and 2 MHz for frequency stability for the two-wavelength seeding operation. Allan variance plot shows that frequency fluctuations are reduced below 70 kHz for 10 s of averaging which is suitable for accurate CO₂ DIAL or IPDA measurements.     

高功率TEA CO2激光器两波长激光切换输出技术

 以TEA CO₂激光器通常采用的平-凹光学稳定腔为基础,提出了一种新的波长选支方法——输出窗口镀膜选支方法。利用一台高平均功率TEA CO₂激光器进行了选支实验研究,结合现有光学镀膜技术,得到了中心波长为9.3 μm的激光单谱线输出,其单脉冲能量及平均功率与激光器原中心波长10.6 μm单谱线输出的相应参数基本相当。研究发现,以相同单脉冲能量激光照射热敏纸时,中心波长9.3 μm激光光斑与中心波长10.6 μm的明显不同。同时,还设计出两波长窗口密闭免调切换装置,在一台激光器上实现了10.6,9.3 μm两个中心波长激光同等功率水平的免调切换输出,切换位置误差小于5″,密封性能满足使用要求。     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

First diode-pumped Yb-doped solid-state laser continuously tunable between 1000 and 1010 nm

We demonstrate that the ytterbium-doped medium Yb:YSO can provide laser oscillation in the 1000–1010 nm wavelength range under diode pumping around 980 nm. Ytterbium-doped Y₂SiO₅ crystal has been chosen for its high emission cross section around 1 m combined with a strong absorption line at 978 nm. We have developed an original pumping architecture that permits us to overcome problems due to the small difference between pump and laser wavelengths. In this way, laser emission continuously tunable in the 1000–1010 nm wavelength range has been obtained with more than 100 mW of output power. This kind of laser is the first step toward a metrological source around 500 nm stabilized on an I₂ absorption line. Furthermore, 100 mW of laser light has been obtained at 998.5 nm, leading to a remarkably low quantum defect of 2.1%. PACS 42.55.Xi; 42.60.Pk; 42.72.Ai     

Diode-pumped passively Q-switched and Q-switch mode-locked Nd:YVO<Subscript>4</Subscript> laser using single-wall carbon nanotube based saturable absorber

We present the performance of diode end-pumped Nd:YVO₄ laser in Q-switched and Q-switched mode-locking oscillation using a single-walled carbon nanotube based saturable absorber, which was fabricated using similar vertical evaporation technique. The average output power, repetition rate and pulse width of the Q-switched laser output were studied with different output couplers. The maximum average output power was 130 mW. For Q-switched mode-locking operation, the repetition rate of the mode-locked pulses concentrated in the Q-switched envelope was 58 MHz. The repetition rate of the Q-switched envelope maintained at 18 kHz, while the pulse width decreased along with the increasing of pump power. The maximum average output power was 53 mW.     

A Q-switched Nd:YAlO3 laser emitting 1080 and 1342 nm

In this paper, the study of a switchable electro-optic Q-switched Nd:YAlO₃ laser emitting 1080 and 1342 nm wavelength was demonstrated. The two-wavelength lasers were eradiated by a Nd:YAlO₃ crystal pumped by a xenon lamp. Two KD*P Pockels cells were adopted as Q-switches. The output energy of 277.2 mJ at 1080 nm wavelength with a pulse width of 20 ns and that of 190.67 mJ at 1342 nm wavelength with a pulse width of 40 ns was obtained, respectively, in our experiment. By switching on or off the voltages applied to the Pockels cells, the operation of the two-wavelength lasers can be selected in free running mode or Q-switching mode.     

Intracavity absorption of CO2 laser radiation by chlorofluorocarbon (CFC) gases

Intracavity absorption method was applied to determine the absorption coefficients of trichlorofluorocarbon CCl₃F (CFC-11), dichlorodifluorocarbon CF₂Cl₂ (CFC-12) and chlorodifluorocarbon CHClF₂ (CFC-22) vs. the pressure in the cell inside the cavity of a tunable CO₂ laser at different spectral lines on branches 9R and 10P.The laser output power was measured vs. the gas pressure at different spectral lines on branches 9R, 9P, 10R and 10P of CO₂ molecule transitions. A strong absorption was observed for lines of 9R and 10P branches, whereas a weak absorption was noticed for lines of 9P and 10R branches.The calculation of absorption coefficients was restricted for 9R and 10P due to the oscillating variation of the output power of CO₂ laser vs. the CFC pressure, which was occurred for the lines of 9P and 10R.On the basis of absorption coefficients, the absorption cross-sections for CFC-12 were calculated and compared with the absorption cross-sections found from the previous experiment (where the cell was located outside the cavity), NIST and HITRAN databases, respectively.The obtained data could be useful for CFC gases detection as pollutants in the atmosphere.     

Room temperature diode-pumped 1875.1 nm Tm:GdVO<Subscript>4</Subscript> laser

A diode end-pumped single-frequency Tm:GdVO₄ laser at room temperature was reported. The maximal output power of single-frequency is as high as 66 mW by using two uncoated fused etalons, which are respectively 0.05 mm thick YAG and 1 mm thick quartz. We obtained the single frequency Tm:GdVO₄ laser at 1875.1 nm. The slope efficiency is 1.5%. The change of the lasing wavelength on temperature was also measured. The single-longitudinal-mode (SLM) laser can be used as a seed laser for coherent wind measurements and differential absorption LIDAR systems.     

High power diode-side-pumped rod Nd:GdVO4 laser

A compact high power diode-side-pumped Nd:GdVO₄ laser has been presented, which can generate an output power of 52 W at 1.063-μm for continuous-wave (CW) operation. The absorption characteristics of the Nd:GdVO₄ in different pump directions is measured, which were used to optimize the diode-side-pumped Nd:GdVO₄ laser head. The laser characteristics of both CW and Q-switched Nd:GdVO₄ and Nd:YAG in are compared and it was found that Nd:GdVO₄ may surpass Nd:YAG for high power laser application.     

Optical properties and laser performance of neodymium doped scheelites CaWO4 and NaGd(WO4)2

Nd-doped CaWO₄ (CWO) and NaGd(WO₄)₂ (NGWO) single crystals with good optical quality have been grown by the Czochralski technique. The neodymium distribution coefficient in these matrices is about 0.4 for CWO and close to unity for NGWO. Polarized absorption and emission spectra have been recorded at room temperature and used to calculate the absorption and stimulated emission cross-sections.1 and 2% Nd : CWO and 1 % Nd:NGWO laser rods have been tested in a cavity longitudinaly pumped by an 1 W AIGaAs laser diode and compared to Nd : YAG and Nd:YVO₄ rods in the same conditions. The 2% Nd: CWO rod exhibits the best performance with slope efficiencies of 64%, higher than in the case of YAG and equal to the YVO₄ samples. The dependence of the laser output power versus the diode temperature has been measured for all these materials. The laser output of Nd: CWO was found to be nearly as stable as for Nd : YVO₄ and much more stable than in the case of Nd : YAG. A CWO microchip has also been tested for the first time to our knowledge.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

diode-pumped single-frequency Tm:GdVO<Subscript>4</Subscript> laser at 1897.6 nm

A diode end-pumped Tm:GdVO₄ laser at room temperature is reported. The maximal output power of single-frequency is as high as 34 mW by using two uncoated fused etalons, which are respectively 0.05 mm thick YAG and 1mm thick quartz. We obtained the single frequency Tm:GdVO₄ laser at 1897.6 nm with the slope efficiency of 1.3%. The single-longitudinal-mode (SLM) laser can be used as a seed laser for coherent wind measurements and differential absorption LIDAR systems.     

Quasi-three-level neodymium vanadate laser operation under polarized diode pumping: theoretical and experimental investigation

We propose a novel technique for pumping neodymium vanadate crystal in ⁴ F _3/2 ?? ⁴ I _9/2 transition with polarized diode light. With a theoretical model on quasi-three-level neodymium vanadate lasers including excited state absorption and energy transfer upconversion effects, the improvement on the laser performance of polarized pumping is evaluated. A maximum output power of 4.8 W in Nd:GdVO₄ 912 nm laser is achieved with the incident pump power of 21.8 W, the maximum output power increases about 85% and the slope efficiency is enhanced to 1.5 times towards the unpolarized pumping under the same condition. This technique is especially suitable for quasi-three-level systems end pumped by high-brightness fiber coupled diode sources associated with short neodymium vanadate crystals.     

High repetition rate LiF<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>: Color center laser

The efficient oscillation of LiF:F₂⁻ color center laser pumped by a compact LD-pumped Nd:YVO₄ acousto optically Q-switched laser with 30 kHz pulse repetition rate was demonstrated. The broadband oscillation with 75 μJ pulse energy and 37 kW peak power with the slope efficiency 20% was obtained. The average output power as high as 230 mW was reached. The narrow line tunable from 1.10 to 1.29 μm laser radiation with 10% conversion efficiency in the maximum of the tuning curve was achieved under pumping with 1.6 W average pump power.     

Continuous-wave high-power multi-pass pumped thin-disc Nd:GdVO4 laser

A thin-disc Nd:GdVO₄ laser in multi-pass pumping scheme was developed. Continuous-wave output power of 13.9 W at 1.06 μm for an absorbed power at 808 nm of 22 W was demonstrated from a 250-μm thick, 0.5-at.% Nd:GdVO₄ in a 4-pass pumping; the slope efficiency in absorbed power was 0.65, or 0.47 in input power. Output performances were also investigated under diode laser pumping at 879 nm, directly into the emitting ⁴F_3/2 level: maximum power of 3.6 W was obtained at 6.2 W of absorbed power with 0.69 slope efficiency. Compared with pumping at 808 nm, into the highly absorbing ⁴F_5/2 level, improvements of laser parameter in absorbed power (increase of slope efficiency, decrease of threshold) were obtained, showing the advantages of the pumping into the emitting level. However, the laser performances expressed vs. the incident power were modest owing to the low absorption efficiency at 879 nm. Thus, increased number of passes of the medium would be necessary in order to match the performances in input power obtained under 808-nm pumping.     

Diode-pumped Tm:GdVO<Subscript>4</Subscript> lasers with double etalons

Diode end-pumped single-frequency Tm:GdVO₄ laser at room temperature was reported. The maximal output power of single-frequency is as high as 51 mW by using two uncoated fused YAG etalons, which are respectively 0.05 mm thick and 1 mm thick. We obtained the single frequency Tm:GdVO₄ laser at 1919.7 nm. The slope efficiency is 1.4%. The single-longitudinal-mode (SLM) laser can be used as a seed laser for coherent wind measurements and differential absorption LIDAR systems.     

Ethylene monitoring by differential absorption with a CO2 waveguide laser

In this paper we report on a differential absorption technique using a CO₂ waveguide laser. The method is based on the measurement of the differential absorption between two different frequencies of the same selected CO₂ laser line. The sensitivity of the technique depends strongly on the total pressure and has been estimated to be about a few tens of ppb or better over a 1 km path length in C₂H₄ diluted with air to a total pressure of a few tens of torrs. Its relative simplicity and high sensitivity at low total pressure enable this technique to be applied to gas detection either at high altitude or to local monitoring of a low pressure sample.