Far-infrared laser emission from deuterated ammonia

By optically pumping the deuterated isotopomers of ¹⁴NH₃ and ¹⁵NH₃ using ¹²C¹⁶O₂, ¹³C¹⁶O₂, ¹²C¹⁸O₂, and ¹³C¹⁸O₂ lasers, several new far-infrared (FIR) emission lines between 65 μm and 125 μm have been detected. The existing spectroscopy of ¹⁴N-ammonia isotopomers has been used to identify many of these lines, as well as some previously observed but unidentified. The spectroscopic data have been analyzed to predict over 20 additional FIR laser lines that could be pumped by a more capable CO₂ laser. This effort was motivated by a need for strong laser lines in frequency coincidence with molecular transitions of astrophysical interest. Of particularnote is the measurement of the 2680-GHz line of ¹⁴NHD₂, whose frequency is 4.9 GHz higher than that of the important J=1-0 line of interstellar HD. Received: 25 July 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/492-5941, E-mail: boreiko@spot.colorado.edu     

Efficient generation of FIR radiation by optical pumping of D2 18O

In this paper we show that D₂ ¹⁸O vapour, optically pumped with a continuously tunable high pressure CO₂ laser, is an excellent source for far infrared radiation. Both high photon conversion coefficients and broad Raman gain regions were found for a large number of new laser transitions spread over the frequency range from 25 cm^–1 to 240 cm^–1. We demonstrate that these Raman gain regions can be used to generate far infrared laser pulses with high intensity and durations of about 100 ps.     

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     

Generation of subpicosecond vacuum ultraviolet pulses at 126 nm by using harmonics of a subpicosecond Ti:Sapphire laser

Subpicosecond vacuum ultraviolet (VUV) pulses at the wavelength of 126 nm have been generated in rare gases as a result of the 7th harmonic radiation of a subpicosecond Ti:Sapphire laser oscillating at 882 nm. The VUV harmonic intensity was optimized in Xe at the pressure of 1.2 Torr. The behavior of the harmonic emission was qualitatively reproduced by the classical nonlinear optics. The increase of the harmonic intensity was limited by multiphoton ionization of Xe.     

Six-Watt mid-infrared laser using harmonic generation with Tl3AsSe3

An output of 6 W at 30 kHz was achieved using Tl₃AsSe₃ as the nonlinear second-harmonic generator. It was pumped with a CO₂ laser source at a wavelength of 9.25 m with 20 ns pulses, and the laser beam was focussed to a spot size of 150 m. The focussed spot was scanned to reduce the continuous-wave power loading, and the system could be operated for extended periods of time under these conditions.     

Elimination of cracking during UV laser ablation of SrTiO3 single crystals by employing a femtosecond laser

We have performed a comparative study of UV laser ablation of SrTiO₃ with nanosecond- and sub-picosecond sources, respectively. The experiments were performed with lasers at a wavelength of 248 nm and pulse durations of 34 ns and 500 fs. Femtosecond ablation turns out to be more efficient by one order of magnitude and eliminated the known problem of cracking of SrTiO₃ during laser machining with longer pulses. In addition, the cavities ablated with femtosecond pulses display a smoother surface with no indication of melting and well-defined, sharp edges. These effects can be explained by the reduced thermal shock effect on the material by using ultrashort pulses.     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

High dynamic range measurement of temporal shape and contrast of ultrashort UV pulses

This paper presents an experimental technique for measurement of the contrast ratio of ultrashort UV pulses. As a multiple-shot device based on the scheme of difference frequency generation this is, to our knowledge, the only cross correlator in the UV so far, which offers a dynamic range of 10⁷ and operates with input pulse energies as low as 5 μJ. By changing the cross correlator into single-shot mode, the temporal shape of the UV pulse can be measured. Received: 26 October 1998 / Revised version: 13 January 1998 / Published online: 28 April 1999     

Low threshold, actively Q-switched Nd:YVO4 self-Raman laser and frequency doubled 588 nm yellow laser

We reported an actively Q-switched, intracavity Nd³⁺:YVO₄ self-Raman laser at 1176 nm with low threshold and high efficiency. From the extracavity frequency doubling by use of LBO nonlinear crystal, over 3.5 mW, 588 nm yellow laser is achieved. The maximum Raman laser output at is 182 mW with 1.8 W incident pump power. The threshold is only 370 mW at a pulse repetition frequency of 5 kHz. The optical conversion efficiency from incident to the Raman laser is 10%, and 1.9% from Raman laser to the yellow.     

High Power Photonic Crystal Fibre Raman Laser

A cw Raman laser based on a 100-m photonic crystal fibre is demonstrated with up to 3.8 W output power at the incident pump power of 12 W, corresponding to an optical-to-optical efficiency of about 31.6%.. The second order Stokes light, which is firstly reported in a cw photonic crystal fibre Raman laser, is obtained at 1183nm with an output power of 1.6 W and a slope efficiency of about 45.7%.     

The mechanism of the CW sodium-vapor/hydrogen laser

Measurements have been carried out to gain information on the mechanism of the cw 9 m sodium-vapor/hydrogen laser [2]. Observations were carried out on the laser excited with both cw and pulsed discharges and the low-power gain of various gas mixtures was measured using tunable diode lasers. The laser was found to oscillate with mixtures of sodium vapor with H₂, D₂ and HD, but with no other gas. On substituting D₂ for H₂ a more than three-fold increase in laser gain was observed. The results showed that most of the processes used to explain lasing action in other metal-vapor lasers were not applicable to the present case. It was concluded that the primary process which leads to the production of the population inversion involves differential quenching of the laser levels by molecular hydrogen, with the lower 3D level being quenched more efficiently than the upper 4P level. Measurements carried out on the calcium/hydrogen laser [21] with D₂ supported these conclusions.     

Folded resonator-dual polarization competition laser displacement sensor

The existing so-called “dual polarization competition laser displacement sensor” applies a cat’s eye resonator. The resonator could be disarranged when the cat’s eye reflector (CER) swings slightly with the rod of the slide rail in measuring the procedure and droops due to its deadweight in long-time operation. This influences the stability of the sensor significantly. In this paper, a novel cat’s eye folded resonator was built, based on which our system named “folded resonator-dual polarization competition laser displacement sensor” was set up. It has a high resolution of λ/16 (39.6 nm) and large measurement range of up to 17 mm. The misalignment sensitivities of traditional cat’s eye resonator and the folded one are both analyzed; and comprehensive experiments are carried out, through which a conclusion can be drawn out that the maximal adjusting angle and the maximal cross displacement of the CER in folded resonator are both about 5 times of those in cat’s eye cavity. Furthermore, the measurement results from this new sensor prove its high stability.     

Selective optothermal detection of NO2 and H2O with a frequency-tuned CO waveguide laser

The results are reported of the CO-laser optothermal (OT) detection of impurity gases when their absorption spectra overlap with those of an interfering gas. The influence of the latter was avoided using low gas pressures corresponding to a maximum of the OT sensitivity. Frequency tuned in the 5.2–6.3 m wavelength range, ¹²C¹⁶O and ¹³C¹⁶O waveguide lasers were used. The fine frequency tuning at 490 MHz was achieved for 150 laser transitions of both molecules. The OT sensitivity was estimated by NO₂ detection in the presence of water vapor. The minimal detectable concentration proved to be 60 ppb at P _19–18(14) transition of a ¹²C¹⁶O laser for NO₂ and 75 ppb on P _12–11(13) transition of a ¹³C¹⁶O laser for H₂O.     

High repetition rate spiker-sustainer XeCl laser

An X-ray preionized spiker-sustainer XeCl-excimer laser operating at high repetition frequency (1000 Hz) and 220 W of average laser power is demonstrated. The pulse energy and the pulse duration (160 ns) are virtually independent of the repetition frequency. Comparison with aC-L-C circuit shows superior properties of spiker-sustainer excitation for high-frequency operation. Furthermore, a variation of the overshoot mode is proposed.     

Misalignment sensitivity of the cat’s eye cavity He-Ne laser

A concave mirror and a cat’s eye reflector acting as a resonator mirror form the cat’s eye cavity. Misalignment sensitivities of the cat’s eye cavity and conventional resonator are both analyzed in geometric method and matrix optics with misalignment sensitivity parameter. Valuable conclusions are drawn: in full-external He-Ne laser, cat’s eye cavity can improve the laser stability up to about 60 times better than the conventional one; diffraction loss introduced by the misalignment of the cat’s eye cavity attributes to the straight-line displacement vertical to the laser bore of the cat’s eye reflector; and with the convex lens center of the cat’s eye reflector secured immobile, the ultra-stable and adjustment-free cat’s eye cavity He-Ne laser is obtained. The analysis matches the experiment results very well. Cavities with three kinds of dimension errors are also calculated. This paper could be used as theoretic foundation for the design and application of cat’s eye cavity lasers.     

Control of transverse mode pattern in a helium-neon laser using the cat’s eye cavity

This paper introduces an innovative method to control and select transverse mode pattern in a He-Ne laser. The cat’s eye cavity is established by using a cat’s eye reflector as the reflecting mirror. Laser transverse mode pattern varies continuously when the distance between the convex lens and the concave mirror is changed, which form the cat’s eye reflector. By Newton-Cotes method, the numerical solution of two-dimensional diffraction integral equation in the cat’s eye cavity is obtained, the laser operating modes under various parameters are analyzed, and the eigenvalue η_mn and diffraction power loss corresponding with every transverse mode are calculated. The parameters assuring fundamental transverse mode output are figured out, which match the experiment results well. Therefore, a new convenient real-time means for the control and selection of the laser transverse mode is realized.     

Chemically pumped O2(a-X) laser

A theoretical and experimental study was conducted aimed at achieving laser oscillation in the (a-X) electronic transition of oxygen molecules. Although this transition is highly forbidden by rigorous selection rules, it may nevertheless concede stimulated emission, if the population inversion is high enough. The idea is based on a recently developed apparatus, namely, a porous pipe type high-pressure chemical singlet oxygen generator. A numerical model which describes the characteristics of this generator was developed to estimate the population inversion and small-signal gain achievable in a laser cavity using this source. The calculations showed that the small-signal gain ought to be sufficient to achieve laser oscillation. Preliminary experiments were conducted, but lasing was not yet observed. It is shown that the scattering losses caused by water droplet aerosols are mainly responsible for preventing our system from laser oscillation.     

Highly efficient Raman conversion in O2 pumped by a seeded narrow band second-harmonic Nd:YAG laser

The first-Stokes conversion efficiency for a stimulated Raman scattering (SRS) is usually very low in gaseous oxygen media. In 3.0 Mpa O₂, a single longitudinal mode second harmonic Nd:YAG laser pump source gives a typical vibrational first-Stokes conversion efficiency of only 2.5%, In comparison, the accompanying stimulated Brillouin scattering (SBS) attains a reflectivity of 67%. However, by seeding an OPO beam into the Raman cavity, the first-Stokes photon conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5% in a 6.1 Mpa 41:59 O₂/ He mixture. This 54% efficiency was obtained for a seeder laser pulse-width less than one half that of pump laser (6.8 ns). A first-Stokes peak power conversion efficiency as high as 88% has been obtained when the pump and seeder pulse peaks coincide. So, we may expect a higher first-Stokes photon conversion efficiency if the seeder pulse-width can be made equal to or larger than that of the pump pulse. On the other hand, the beam quality of the first-Stokes in an O₂/ He mixture excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ and a pump laser repetition rate of 10 Hz, the thermal defocusing effect worsens the first-Stokes beam quality. This thermal defocusing effect is a result of the Raman heat release and could be eliminated by fast circulating and cooling the Raman gas medium.     

Threshold and conversion experiments on seeded stimulated Raman scattering

High energy-conversion efficiencies in Stimulated Raman Scattering (SRS) are demonstrated both in experiments and by simulations for pump powers below SRS threshold. The scattering is induced by a short seed pulse at the Stokes frequency, the pulse width of which is much shorter than the pump pulse width and which is comparable with the medium's dephasing time.     

Far-infrared signature of animal tissues characterized by terahertz time-domain spectroscopy

We present terahertz time-domain spectroscopy characterization of various animal tissues obtained from pork and rats. As the sensitivity of terahertz radiation to polar molecules of water is very high, biological tissues with high level of hydration show strong absorption at terahertz frequencies. The experimental data indicate that skin, fat and lean pork tissues have different frequency-dependent response to terahertz radiation due to the variation in water content. The same type tissue from different animals, however, is observed to show very similar water absorption.