Laser diode spectroscopy of H2O at 2.63 μm for atmospheric applications

The 2.7 μm spectral range is highly suitable for the in situ monitoring of atmospheric H₂O using compact balloonborne laser diode spectrometers. Water vapour spectroscopic parameters of the 2₀₂   1₀₁ and the 4₁₃   4₁₄ transitions of the ν₃ band are revisited in this spectral region using a new distributed-feedback InGaAsSb laser diode emitting at 2.63 μm. Accurate line strengths are provided which are well adapted for the in situ probing of the middle atmosphere. Our measurements are thoroughly compared to an existing molecular database, laboratory measurements and ab-initio calculations. A laser hygrometer was developed for operation from small stratospheric balloons using this new laser diode technology, with emission at 2.6 μm. The realized sensor is described and results from a recent test-flight are reported. PACS 07.57.Ty; 92.60 Jq     

Free-running and Q-switched operation of a diode pumped Er:YSGG laser at the 3 μm transition

A study is made of a diode pumped Er³⁺:YSGG laser crystal operating at 2.797 μm. Lasers were constructed in the bounce geometry, using a transversely cooled 50 at.% Er:YSGG slab and a face-cooled 38 at.% Er:YSGG slab. Results from these are compared with those from a 50 at.% Er³⁺:YAG laser, also in the bounce geometry. With quasi-continuous wave diode pumping, free-running pulse energies of up to ~55 mJ and a slope efficiency of 20.5% are obtained from 50 at.% Er:YSGG. Better thermal performance is obtained from the face-cooled 38 at.% Er:YSGG slab, allowing average power of ~2 W to be obtained at a repetition rate and pump pulse duration of 140 Hz and 500 μs, respectively. Both Er:YSGG systems perform better than Er:YAG. Numerical modelling of the free-running 50 at.% Er:YSGG and Er:YAG systems is undertaken with good qualitative agreement with experimental results. Electro-optic Q-switching of the 50 at.% Er:YSGG laser using a LiNbO₃ crystal yields ~0.5 mJ pulses with ~77 ns duration.     

Low-cost low-threshold diode end-pumped Tm:YAG laser at 2.016?μm

We report a low-threshold continuous-wave Tm:YAG laser that can be excited near 785?nm with low-cost, single-mode AlGaAs laser diodes. Low-threshold operation was achieved using a tightly focused, astigmatically compensated x-cavity containing a 2-mm-thick Tm:YAG crystal with 5?% Tm³⁺ concentration. Two linearly polarized single-mode diodes operating at 785.8?nm were polarization coupled to end pump the resonator. With a 6?% output coupler, as high as 32?mW of output power could be obtained at 2016?nm with 184?mW of incident pump power. The output could be further tuned in the 1935?C2035?nm range. Slope efficiency measurements indicated that cross-relaxation was very effective at this doping level. With a 2?% output coupler, lasing could be obtained with as low as 32.3?mW of pump power. In the limit of vanishing output coupling, the incident threshold pump power could be reduced to as low as 25?mW. To our knowledge, this is among the lowest lasing thresholds reported to date for continuous-wave, room-temperature thulium lasers.     

Detection of HCl on the first and second overtone using semiconductor diode lasers at 1.7 μm and 1.2 μm

Sensitivity studies are also performed, to evaluate the minimum detectable concentration of HCl in air. Received: 7 August 1998/Revised version: 5 October 1998     

Passive Q-switching of 1.44 μm and 1.34 μm diode-pumped Nd:YAG lasers with a V:YAG saturable absorber

Spectroscopic data of a V³⁺:YAG passive Q-switch crystal were measured. The absorption recovery time was determined to be of 37±7 ns and the ground state absorption cross section was estimated to be 0.7×10^-18 cm² at 1.44 μm and 3.5×10^-18 cm² at 1.34 μm. Passively Q-switched operation of diode pumped 1.44 μm and 1.3 μm Nd:YAG lasers was demonstrated using this crystal as saturable absorber. Average output powers of 1.42 W (1.44 μm) and 1.56 W (1.34 μm) and pulse energies of 24 μJ (1.44 μm) and 25 μJ (1.34 μm) were observed, respectively. Received: 19 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6281, E-mail: kretschmann@physnet.uni-hamburg.de     

Performance of a diode end-pumped Cr,Er:YSGG laser at 2.79 μm

We present a diode end-pumped Cr,Er:YSGG laser at 2.79 μm operated in the continuous-wave (CW) and pulsed modes. Er³⁺ ions are directly pumped into the upper laser state ⁴ I _11/2 using a 967-nm diode laser, which avoids various nonradiative losses. The laser produces 50.2 mW of power in the CW mode. An average power of 169 mW is obtained at a repetition rate of 100 Hz and a pulse duration of 2 ms. The maximum laser energy achieved is 1.95 mJ at a repetition rate of 50 Hz, corresponding to a peak power of 970 mW and a slope efficiency is 7.6%. The far-field divergence angle of the laser is measured to be 8 mrad.     

Study of passive Q-switching for flash-lamp-pumped 1.34-μm Nd:GdVO_4 laser

The static laser performance of a-growth Nd:GdVO4 crystal (a-cut, 4×4×25 (mm)) at 1.34μm pumped by flash-lamp is investigated with different transmissions of output couplers. With the output coupler transmission of T = 30%, the static output energy of 148 mJ is obtained when the pump energy is 35.2 J, and the corresponding electric-optical conversion efficiency is 0.42%. The Q-switched output of lasers with the output wavelength ranging from 1.3 to 1.6μm can be realized by using Co2 :LaMgAl11O19 (Co:LMA) as saturable absorber. A flash-lamp-pumped, passively Q-switched Nd:GdVO4 laser with Co:LMA as saturable absorber is demonstrated in plano-concave laser cavity. With the cavity length of 16.3 cm and pump energy of 19.8 J, the single-pulse output energy, pulse width, and peak power are obtained to be 4 mJ, 80 ns, and 5×104 W, respectively.     

高效率中红外2.7μm可调谐激光器

报道了采用1064nm激光抽运周期极化掺杂氧化镁的铌酸锂(PPMgO…CLN)晶体光参变振荡器(OPO)实现高效率2.7μm可调谐激光输出的实验结果,理论计算了PPMgO…CLN晶体准相位匹配(QPM)温度调谐曲线。在1064nm激光抽运功率为182W,声光Q开关工作频率为10kHz和PPMgO:CLN晶体工作温度为150℃条件下,获得了平均功率为36.7W、波长为2.73μm中红外激光输出,斜率效率为23.7%。通过改变周期为31.3μmPPMgO…CLN晶体的工作温度30℃～200℃,获得了中红外波长3.0～2.6μm激光输出,波长温度调谐实验结果与理论分析结果有大约10℃的差异。     

Efficient 2.7-μm emission in Er3＋-doped bismuth germanate glass pumped by 980-nm laser diode

An Er 3+-doped lead-free bismuth germanate glass is synthesized. Its 2.7-μm emission property is analyzed, and the efficient 2.7-μm emission from the glass is observed under 980-nm laser diode excitation. The prepared glass possesses high spontaneous transition probability (64.8 s 1 ) and a large calculated emission cross section (6.61×10 21 cm 2 ) corresponding to the 4I11/2→4I13/2 transition. The multiphonon relaxation rate for the excited state 4I11/2 is only 236 s 1 . Therefore, the excellent spectroscopic properties and outstanding thermal stability suggest that this glass is a suitable host for developing solid-state lasers operating in the mid-infrared.     

Line strength and collisional broadening coefficients of H2O at 2.7 μm for natural gas quality assurance applications

We employed tunable diode laser absorption spectroscopy to measure the line strength, the methane (CH₄), ethane (C₂H₆) and the propane (C₃H₈) broadening coefficients for the 523–422 H₂O transition at 3619.61 cm^?1. Water amount fractions generated by a stable and accurate humidity transfer standard, traceable to the SI units via the German national humidity standard, were used to calibrate the spectroscopic line strength measurements. We focus on the traceability of the measured line data to the SI and on uncertainty assessments following the guidelines of the Guide to the Expression of Uncertainty in Measurement. We determined the line strength to be (8.42 ± 0.07)×10^?20 cm^?1/(cm^?2 molecule) corresponding to a relative uncertainty of ±0.8%. To the best of our knowledge, we report the first methane, ethane and propane broadening coefficients of (8.037 ± 0.056)×10^?5 cm^?1/hPa, (9.077 ± 0.064)×10^?5 cm^?1/hPa and (10.469 ± 0.073)×10^?5 cm^?1/hPa for the 523–422 H₂O transition at 3619.61 cm^?1, respectively. The relative combined uncertainties of the stated CH₄, C₂H₆ and C₃H₈ broadening coefficients are in the ±0.7% range.     

3.5 μm optical communication experiment at 100 Mbits−1 using a PbCdS diode laser

A mid-infrared optical communication experiment using a lead-salt diode laser emitting at 3.5 m is reported. Bit-error rate measurements have been performed for pulse code modulated data at 100 Mbits^–1 using a fluoride glass fibre as a transmission channel. A bit-error rate better than 10^–10 was achieved at a signal-to-noise ratio of 20 dB.S. T. Eng has a joint appointment with the Jet Propulsion Laboratory, Pasadena, California 91109, USA.     

Compression of picosecond optical pulses from 1.5μm gain switched In GaAsP DFB diode laser

The experimental study of compression of picosecond optical pulses from a gainswitched InGaAsP DFB laser at 1.5μm wavelength range using a Gires-Tournois interferome-ter has been reported. Ultrashort optical pulses with pulsewidths of 6.6 ps, repetition rate of2.4GHz, and peak power in excess of 200 mW are obtained. The characteristics of a Gires-Tournois interferometer as a pulse compressor are also discussed.     

High-Power Operation of Uncoated Strain-Compensated Quantum Cascade Lasers at 4.8μm

High-power operation of uncoated 22μm-wide quantum cascade lasers （QCLs） emitting at λ～ 4.8μm is reported. The emitting region of the QCL structure consists of a 30-period strain-compensated Ino.68 Gao.32 As/In0.37 Alo.63 As superlattice. For a 4-mm-long laser in pulsed mode, a peak output power is achieved in excess of 2240mW per facet at 81K with a threshold current density of 0.64 kA/cm^2. The effects of varying the cavity lengths from 1 to 4 mm on the performances of the QCLs are analysed in detail and the low waveguide loss of only about 1.4 cm^-1 is extracted.     

High Resolution and High Sensitivity Measurement of Methane at 1.51μm

The high-resolution absorption spectrum of CH4 at 1.51μm is observed by direct absorption spectroscopy technique with a White absorption cell. Multi-peak fitting technique is adopted to reveal line positions and line intensities of CH4 from 6608cm^-1 to 6625 cm^-1. Special attention is paid on the determination of the line positions, and the accuracy is better than ±0.002cm^-1. A minimum measurable absorption of 2.1×10^-8 （3σ） has been achieved based on the measured direct absorption spectroscopy.     

Distinction investigation of InGaAs photodetectors cutoff at 2.9 μm

Using double heterojunction structure with linearly graded In_xAl_1–xAs as buffer layer and In_0.9Al_0.1As as cap layer, wavelength extended In_0.9Ga_0.1As detectors with cutoff wavelength of 2.88 μm at room temperature have been grown by using gas source molecular beam epitaxy, their characteristics have been investigated in detail and compared with the detectors cutoff at 2.4 μm with similar structure as well as commercial InAs detectors. Typical resistance area product R₀A of the detectors reaches 3.2 Ω cm² at 290 K. Measured peak detectivity reaches 6.6E9 cm Hz^1/2/W at room temperature.     

2.7～3.0μm波段高反镜反射率测量研究

中红外激光领域广泛使用高性能高反射光学元件,高反射率高精度测试技术是制备高性能反射光学元件的基础。针对2.7～3.0μm波段光学元件高反射率测量的实际需求,基于量子级联激光器建立了连续光腔衰荡反射率测试实验装置,通过优选2.7～3.0μm波段反射带内水汽吸收较弱的测试波长,分析空气中水汽吸收对衰荡时间和反射率测量的影响,并比较空气和氮气环境下反射率测量结果,实现了2.7～3.0μm波段高反镜反射率的准确测量,在反射率约99.95%时绝对测量精度优于2×10-5。实验结果显示,采用测试波长2.9μm并在测量时保证初始腔和测试腔腔长相同,无需使用氮气环境,直接在实验室空气环境可实现高反射率的精确测量。     

Temperature and water concentration measurements in combustion gases using a DFB diode laser at 1.4 μm

A sensor using a single DFB diode laser at 1.4 μm based on wavelength modulation spectroscopy for the measurements of the gas temperature and the H₂O concentration in combustion gases is developed. A line pair of H₂O absorption transitions located at 7085.251 and 7085.876 cm^?1 is selected based on some design rules. The 1f normalized 2f method is used to remove the need for the calibration and to correct for the transmission variation due to beam steering, mechanical misalignments, soot, and window fouling. The precision for the temperature and H₂O concentration measurements are 1.05 and 2.10% in a controlled static cell, respectively. Burner experiments demonstrate the ability of the system for in situ measurements.