The optimum configuration for the third-harmonic generation of 1.064 μm in a YCOB crystal

A method of determining the spatial configuration of a YCa₄O(BO₃)₃ (YCOB) crystal is introduced. The type-I phase-matching points for the sum-frequency of 1.064 μm and 0.532 μm, as well as the conversion efficiency for YCOB samples cut in different directions, have been measured. When the input intensity is 0.76 GW/cm², the third-harmonic generation (THG) conversion efficiency for the (106°, 77.2°)-cut YCOB crystal reaches 26.2%, which is the highest that we know so far. Both the calculations and the experiments show that the optimum THG configuration for a YCOB crystal is near (106°, 77.2°), or other equivalent directions in space. Received: 11 September 2000 / Revised version: 25 January 2001 / Published online: 27 April 2001     

Development of an external cavity quantum cascade laser spectrometer at 7.5 μm for gas detection

We report the development of an external cavity quantum cascade laser spectrometer at 7.5 μm. The quantum cascade laser and its anti-reflection coating were specially developed for this application. We provide details of the external cavity design and data processing. A continuous wave emission is demonstrated from 1,293 up to 1,350 cm^?1. A preliminary test of the spectrometer was realized by measurements on acetone and phosphoryl chloride.     

Development of an OPO system at 1.57 μm for integrated path DIAL measurement of atmospheric carbon dioxide

Active remote sensing is a promising technique to close the gaps that exist in global measurement of atmospheric carbon dioxide sources, sinks and fluxes. Several approaches are currently under development. Here, an experimental setup of an integrated path differential absorption lidar (IPDA) is presented, operating at 1.57 μm using direct detection. An injection seeded KTP-OPO system pumped by a Nd:YAG laser serves as the transmitter. The seed laser is actively stabilized by means of a CO₂ reference cell. The line-narrowed OPO radiation yields a high spectral purity, which is measured by means of a long path absorption cell. First measurements of diurnal variations of the atmospheric CO₂ mixing ratio using a topographic target were performed and show good agreement compared to simultaneously taken measurements of an in situ device. A further result is that the required power reference measurement of each laser pulse in combination with the spatial beam quality is a critical point of this method. The system described can serve as a testbed for further investigations of special features of the IPDA technique. PACS  42.65.Yj; 42.68.Wt; 92.60.hg     

An all-solid-state laser system for generation of 100 μJ femtosecond pulses near 625 nm at 1 kHz

Frequency doubling the output of a high-power femtosecond Cr:forsterite regenerative amplifier with >50% conversion efficiency in a temperature-tuned noncritically phase-matched LBO crystal produces femtosecond pulses of >100 μJ energy in the visible range near 625 nm at a pulse duration of about 200 fs or >65 μJ at <170 fs. Received: 29 March 1999 / Revised version: 27 April 1999 / Published online: 24 June 1999     

Comparative theoretical analysis of continuous wave laser cutting of metals at 1 and 10 μm wavelength

We present a derivation and, based on it, an extension of a model originally proposed by V.G. Niziev to describe continuous wave laser cutting of metals. Starting from a local energy balance and by incorporating heat removal through heat conduction to the bulk material, we find a differential equation for the cutting profile. This equation is solved numerically and yields, besides the cutting profiles, the maximum cutting speed, the absorptivity profiles, and other relevant quantities. Our main goal is to demonstrate the model’s capability to explain some of the experimentally observed differences between laser cutting at around 1 and 10 μm wavelengths. To compare our numerical results to experimental observations, we perform simulations for exactly the same material and laser beam parameters as those used in a recent comparative experimental study. Generally, we find good agreement between theoretical and experimental results and show that the main differences between laser cutting with 1- and 10-μm beams arise from the different absorptivity profiles and absorbed intensities. Especially the latter suggests that the energy transfer, and thus the laser cutting process, is more efficient in the case of laser cutting with 1-μm beams.     

Thermally tunable double-peak phase-conjugate reflectivity in InP:Fe at 1.32 μm

We report on double-peak thermally tuned reflectivity of optical phase conjugation in InP:Fe under dc fields. A single-pump four-wave mixing geometry generates a phase-conjugate reflectivity up to 6.7% for a pump intensity of 28 mW/cm² at 1.32 μm wavelength. The phase-conjugate reflectivity exhibits strong temperature dependence that is attributed to the phase-shift variations of the index grating due to optical power density of the intensity distribution. Two strong peaks in phase-conjugate reflectivity are observed near room temperature and are separated by 3 °C. Our theoretical predictions, which are based on the modified single-defect model, are in good agreement with the experimental findings. Received: 6 December 2000 / Revised version: 13 February 2001 / Published online: 27 April 2001     

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     

Estimation of required parameters for detection of small smoke plumes by lidar at 1.54 μm

Parameters of eyesafe lidar at 1.54 μm for detection of small plumes of smoke from burning wood or oil have been evaluated. It was assumed that a diode-pumped solid-state Er:glass laser at 1.54 μm or a Nd:YAG laser with a Raman cell or optical-parametric oscillator is used as a light source and that detection of backscattered light is performed with an avalanche photodiode. Ash and soot particle size distributions were taken from experiments. A backscattering coefficient at 1.54 μm for various source of smoke was estimated. In computing the laser energy, range between lidar and smoke, receiver optics diameter, fuel mass burned in unit time, fire source radius, laser pulse duration and visibility were varied. Results of the computations enabled estimation of the required laser energy, which ranges from 0.05 to 1400 mJ depending on the parameters. Received: 5 January 2000 / Revised version: 3 March 2000 / Published online: 11 May 2000     

Efficient intracavity second-harmonic generation at 1.06 μm in a BiB3O6 (BIBO) crystal

We report, for the first time, efficient intracavity second-harmonic generation (SHG) at 1.06 μm in a non-linear optical crystal, BiB₃O₆ (BIBO), at a type-I phase-matching direction of (θ,ϕ)=(168.9°,90°), performed with a LD end-pumped cw Nd:YVO₄ laser. A cw SHG output power of 364 mW has been obtained using a 1.9-mm-thick BIBO crystal. The optical conversion efficiency was 12.2% and the corresponding effective intracavity SHG efficiency was determined to be 32.4%. It was found that the intracavity SHG efficiency is greater than that obtained with a type-II phase-matching KTiOPO₄ (KTP) crystal with a thickness of 3 mm. The effective non-linear optical coefficient (d_eff) ratio of BIBO to that of KTP, determined experimentally, is 1.23:1. Received: 7 May 2001 / Revised version: 6 July 2001 / Published online: 19 September 2001     

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO₂ pump power and pulse duration requirements for Raman oscillation. The CO₂ laser oscillation with circular polarization was realized by seeding externally circularly polarized CO₂ radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters. Received: 18 November 1998 / Published online: 26 May 1999     

Raman continuum generation at 1.0–1.3 μm in passively mode-locked fiber laser based on nonlinear polarization rotation

We experimentally demonstrate cascaded Raman scattering continuum generation at 1.0–1.3 μm with pulse energy of 47 nJ by utilizing a commercially available all-normal-dispersion single-mode fiber (SMF) with a low threshold pump power. We achieve passively mode-locked all-fiber lasing by nonlinear polarization rotation technique. Continuous Raman gain is obtained by the same SMF in every roundtrip because of the ring-cavity configuration, which makes it possible to achieve high-order Raman Stokes waves by a short fiber length. Limited by the pump power, the maximum output average power and peak power are 87.4 and 339 mW, but it has great potential to be improved. These make it an ideal continuum source for many applications, such as optical coherence tomography.     

Spectroscopic properties of 1.8 μm emission of thulium ions in germanate glass

A new type host of germanate glass (GeO₂− BaO−BaF₂−Ga₂O₃−La₂O₃) codoped with Tm₂O₃ has been investigated for application as laser material. It possesses a large emission cross section with the value of 9.3×10⁻²¹ cm² at 1.8 μm. Judd-Ofelt intensity parameters and radiative transition probability are calculated and analyzed by Judd-Ofelt theory and absorption spectra. The infrared emission spectra at 1.8 μm have been obtained by using a 794 nm laser diode as excitation resource. The emission intensity ratio of 1.8 (³F₄→³H₆) to 1.47 μm (³H₄→³F₄) increases, while the experimental lifetime of the Tm³⁺:³H₄ level decreases by increasing Tm₂O₃ concentration, which is attributed to the presence of a cross relaxation process. The most intensive emission at 1.8 μm is achieved from the germanate glass with the concentration of Tm₂O₃ reaches 1.0 wt%. The extended overlap integral method is used to calculate the microparameter of the energy transfer and the critical distance, which are derived to better understand the energy transfer process of thulium ions in the germanate glass responsible for emission at 1.8 μm.     

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     

Design of efficient single-stage chirped pulse difference frequency generation at 7 μm,driven by a dual wavelength Ti:sapphire laser

We present simulations for a design of a high-energy single-stage mid-IR difference frequency generation adapted to a two-color Ti:sapphire amplifier system. The optimized mixing process is based on chirped pulse difference frequency generation (CP-DFG), allowing for a higher conversion efficiency and reduced two-photon absorption losses. The numerical start-to-end simulations include stretching, chirped pulse difference frequency generation and pulse compression. Realistic design parameters for commercially available nonlinear crystals (GaSe, AgGaS₂, LiInSe₂, LiGaSe₂) are considered. Compared with conventional unchirped DFG directly pumped by Ti:sapphire technology, we predict a threefold increase in the quantum efficiency. Our CP-DFG scheme provides up to 340 μJ pulse energy directly at 7.2 μm when pumped with 8 mJ and supports a bandwidth of up to 350 nm. The resulting 240 fs mid-IR pulses are inherently phase stable.