CW laser operation of Nd:YLF

CW laser operation of Nd:YLF is reported for the first time and, in a comparison to Nd:YAG, exhibits a lower threshold and higher single-mode average power. The TEM00mode volume of Nd: YLF was observed to be a factor of four larger than Nd:YAG, resulting in twice the TEM00mode average power. This result is attributed to the substantially lower thermal lensing of YLF with respect to YAG. In comparative measurements the stimulated emission cross section of Nd:YLF was found to be1.8 times 10^{-19}cm²for the π oscillation and1.2 times 10^{-19}cm²for the σ oscillation, as compared to2.4 times 10^{-19}cm²for Nd:YAG. The spectroscopic and lasing parameters of Nd:YLF indicate a potential for this material in high peak and average powerQ-switched applications.     

A flashlamp-pumped, Q-switched Cr:Nd:GSGG laser

A long-pulse Cr:Nd:GSGG laser was operated at almost 2 J output energy and almost 5 percent efficiency, using an uncooled pump cavity. An Nd:YAG rod in the same pump cavity produced 3.75 percent efficiency. Using a water-cooled pump cavity of standard design, the thermal focusing of Cr:Nd:GSGG was found to be almost six times that of Nd:YAG for the same flashlamp input energy. The thermal birefringence of the GSGG was also observed to be significantly higher than that of YAG. The performance of aQ-switched Cr:Nd:GSGG laser was characterized and compared to the predictions of a mathematical model for the laser. This yielded an estimate of4.2 times 10^{-19}cm²for the peak stimulated emission cross section of Nd³⁺in GSGG. Using the same technique for Nd:YAG yielded a value of9 times 10^{-19}cm².     

Optimum design for high-power and high-efficiency GaAs Hi-Lo IMPATT diodes

Optimum structure parameters of GaAs hi-lo IMPATT diodes to give a high-efficiency and a high-output power are experimentally determined in the frequency range from 7 to 18 GHz. The highest efficiency is obtained at any frequency when a diode has a punch-through factor of about 0.6 and is depleted with an electric field strength of 10 kV/cm at the end of the lo region under operation. The optimum lo-region carrier concentration to achieve the highest efficiency and the upper limit of a junction area to obtain a high-output power without decreasing efficiency are found to be simply related to frequency by the following equations:(N_{L})_{opt} = (4 times 10^{14}) cdot (f/7)^{3}cm^-3and(S_{j})_{max} = (20 times 10^{-4}) cdot (f/10)^{-1.9})cm²), respectively, wherefis in gigahertz.     

Tunable alexandrite lasers

Wavelength tunable laser operation has been obtained from the solid-state crystal alexandrite (BeAl2O4:Cr³⁺) over the continuous range from 701 to 818 nm. The tunable emission was observed at room temperature and above in a homogeneously broadened, vibronic, four-level mode of laser action. In this mode the laser gain cross section increases from7 times 10^{-21}cm²at 300K to2 times 10^{-20}cm²at 475K, which results in improved laser performance at elevated temperatures. Efficient 2.5 percent, low-threshold (10 J) operation has been obtained with xenon-flashlamp excitation of the 6 mm diameter × 76 mm length laser rods. Output pulses of greater than 5 J and average power outputs of 35 W have been demonstrated, limited by the available power supply. The emission is strongly polarizedEparallelb, with a gain that is 10 times that in the alternate polarization. The 262 μs, room-temperature fluorescence lifetime permits effective energy storage andQ-switched operation. TunableQ-switched pulses as large as 500 mJ have been obtained with pulsewidths ranging between 33 and 200 ns depending on the laser gain. Laser action has also been demonstrated on the high-gain (3 times 10^{-19}cm²emission cross section)Rline at 680.4 nm and is also polarizedEparallelb. This three-level mode is analogous to the lasing in ruby except that the stimulated emission cross section in alexandrite is ten times larger than for ruby.     

Antenna impedance measurements and sheath effects in an RF generated plasma

A device using an RF discharge quasi-homogeneous plasma to measure antenna impedance is described. The plasma diagnosis is done by using a new probe made of a transparent resonant cavity. The electron densities (2 times 10^{8}to6 times 10^{9}particles/cm³) and collision frequencies (6 times 10^{7}to1.5 times 10^{8}per second) are controlled by the discharge power. The density distribution measurements indicate a quasi-homogeneous region of 20 by 20 by 14 cm. Impedance measurement results are given for a a thick unipole between 100 and 750 MHz. The values ofX = (omega_{p}/omega)^{2}range from 0 to 44. They agree fairly well with Deschamps model theory applied to scaled frequency measurements as well as to a quasistatic simple analytical formula. Sheath effects enhanced by a dc bias are observed especially at low frequencies and the sheath thickness increase is calculated and found consistent with Pavkovich's parabolic potential assumption. The possibility of electronic tuning by a suitable bias is suggested.     

Experiments on a semiconductor laser pumped rubidium atomic clock

Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (⁸⁷Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (⁸⁷Rb/⁸⁵Rb =frac{3}{7}) and buffer gases) and gas cell B (⁸⁷Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 times 10^{-12} tau^{-1/2}and2.7 times 10^{-12} tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level⁸⁷Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved⁸⁷Rb atomic clock.     

Electrooptical effects in silicon

A numerical Kramers-Kronig analysis is used to predict the refractive-index perturbations produced in crystalline silicon by applied electric fields or by charge carriers. Results are obtained over the1.0-2.0 mum optical wavelength range. The analysis makes use of experimental electroabsorption spectra and impurity-doping spectra taken from the literature. For electrorefraction at the indirect gap, we findDelta n = 1.3 times 10^{5}atlambda = 1.07 mum whenE = 10^{5}V/cm, while the Kerr effect givesDelta n = 10^{-6}at that field strength. The charge-carrier effects are larger, and a depletion or injection of 10¹⁸carriers/cm³produces an index change ofpm1.5 times 10^{-3}atlambda = 1.3 mum.     

Ellipse rotation studies in laser host materials

Using a TEM00qnear Gaussian mode ruby laser system we report the first experimental measurements of intensity induced changes of optical polarization (ellipse rotation) in a cubic crystalline medium, YAG, for which we obtain the nonlinear susceptibilitieschi_{3}^{1221} (- omega, omega, omega, -omega) = 6.34 times 10^{-15}ESU andfrac{1}{2} (chi_{3}^{1111} + chi_{3}^{1221} - 2chi_{3}^{1212}) = 7.18 times 10^{-15}ESU, accurate to better than ±7 percent relative tochi_{3}^{1221} (- omega,omega,omega, -omega)for liquid CS2. These values are compared with further results obtained for fused quartz and two laser glasses. Moreover, by time resolving the ellipse rotation data we demonstrate the capability to plot ellipse rotation versus input power on a single laser shot, thus increasing the practical feasibility of the technique and introducing the possibility of resolving transient contributions to the measurement.     

Spectroscopy and lasing of a high Nd concentration Al-phosphate glass

An Al-phosphate glass containing Nd³⁺in concentrations ranging from3 times 10^{20}to2.7 times 10^{21}ions/cm³has been prepared and investigated spectroscopically. Different ways of measuring the⁴F3/2→⁴I9/2emission cross sections yield values between1.96 times 10^{-20}and2.7 times 10^{-20}cm². At the highest concentration the decay time is 50 μs, while the radiative lifetime is estimated to begeq446 mus. Lasing experiments were performed by pumping thin platelets of glass coaxially with a dye laser. Cross sections, losses, and differential efficiencies are derived from these experiments. The status of high concentration glasses versus stoichiometric Nd compounds is reviewed.     

Emission cross section and fluorescence efficiency of Nd-pentaphosphate

The room-temperature cross sections for the Nd³⁺⁴F_{3/2}levels to the⁴I_{11/2}and⁴I_{9/2}manifolds (lower laser state and ground state, respectively) in NdP5O14are measured by two spectroscopic methods. A value for the largest cross section ofsigma(R_{1} - Y_{2}) = 1.7 times 10^{-19}cm²is found. The highest effective cross section, resulting from superposition of two lines at 1.051 μm, gives a laser gain per Nd ion which is about 2/3 of the maximum gain in YAG:Nd. The relative branching ratio into the⁴I_{11/2}and⁴I_{9/2}manifolds is 0.65:0.35. Comparison of the integrated cross sections with the measured lifetime for 1-percent Nd in LaP5O14indicates a combined efficiency <0.1 for the remaining transitions, namely radiative decay into the⁴I_{13/2}and⁴I_{15/2}manifolds and multiphonon quenching. A measurement of temperature dependence of fluorescence lifetime supports this last result.     

Absorption in the near-ultraviolet wing of the Kr2F*(410 nm) band

An intracavity laser technique has been used to study the absorption of electron-beam pumped Ne/Kr/F2gas mixtures (196 and 300 K) in the "blue wing" of the Kr2F emission continuum. The experiments were conducted at 358 nm using theupsilon' = 0 rightarrow upsilon" = 1transition of the N2(C rightarrow B) laser. Comparing the results with the predictions of a computer model, the species primarily responsible for absorption have been identified as Ne+2, Kr+2, and Kr2F*. The photoabsorption cross sections for Ne+2and Kr2F (Kr+2F-) at 358 nm have been estimated to be8.1 cdot 10^{-19}and5.4 cdot 10^{-18}cm², respectively. The Kr2F* absorption cross section is roughly 20 percent of that reported for Kr+2at the same wavelength. The fluorescence efficiency of Kr2F* ine-beam excited 94.93 percent Ne/5 percent Kr/0.07 percent F2(P_{total} = 4000torr) gas mixtures has been found to be a factor of 2.8 higher than that of the N2(C rightarrow B) band in Ar/5 percent N2mixtures. Also, the rate constant for quenching of Kr2F* by F2was measured to be(4.1 pm 0.5) cdot 10^{-10}cm³. s^-1at 300 K and(3.0 pm 0.5) cdot 10^{-10}cm². s^-1at 196 K.     

Amplification cross section of the 1.052-µ transition of Nd3+in POCl3- SnCl4-(H2O) system by three different methods

We report the measurement of the amplification cross section σ21of the Nd³⁺ion in the POCl3-SnCl4-(H2O) system by three different methods. The first is based on the simultaneous measurement of the variation of fluorescence andQ-spoiled laser emission. The second is purely spectroscopic. The third is based on the measurement of k12at several temperatures. The two last methods give similar results. The most probable value issigma_{21} = 8.5 times 10^{-20}cm². An explanation for the lowest valuesim 6 times 10^{-20}cm²found with the first method is given.     

Study of the XeCl laser pumped by a high-intensity electron beam

We present the results of a detailed experimental study of the XeCl laser pumped by a high-intensity electron beam. The laser system was optimized as an oscillator for mixtures of Xe and HCl with Ne, Ar, and Kr diluents. The peak intrinsic efficiency (laser energy out/electron-beam energy deposited) was near 4.5 percent for each of these diluents. Small-signal gain and background absorption were measured as a function of electron-beam deposition rate from 0.4 to 6 MW/ cm³. The ratio of small-signal gain to absorption was found to be constant over this range with a value of ∼5. Measurements of absorption in the presence of a large photon flux indicated that there was no appreciable saturable contribution to the absorption. Measurements of fluorescence from theBandCstates indicate that collisional mixing between these states is very rapid. The formation efficiencies of theBandCstates are estimated to be 0.15 and 0.05, respectively. A vibrational relaxation rate of between 1 and1.5 times 10^{-10}cm³. s^-1was determined. The effect of this finite relaxation rate is to reduce the energy available to the stimulated process by a factor of 0.67-0.75. Estimates of the XeCl* deactivation rates by HCl and electrons were also obtained. A value of1.7 times 10^{-9}cm³. s^-1was obtained for quenching by HCl, and a value ofsim 1 times 10^{-7}cm³. s^-1was estimated for electron deactivation.     

Analyses of mode-hopping phenomena in an AlGaAs laser

Intensity fluctuations of the longitudinal modes of a 0.8 μm AlGaAs laser were precisely measured during the occurrence of hopping between two modes. It was found from this result that mode hopping follows the stochastics of a Poisson process. The frequency of mode hopping was measured asf_{c} = [exp [-95(I/I_{th} - 1)]] times 10^{7}(Hz). whereI/I_{th}is the injection current normalized to its threshold value. Results of analog computer simulations showed that spontaneous emission worked as a triggering force for mode hopping. Results of the analysis based on the Fokker-Planck equation were compared to the experimental results, from which the root-mean-square value of the fluctuating electric field of spontaneous emission was estimated as2.3 times 10^{2)(V/m)leqlanglesim{E}_{N} leq 3.2 times 10^{2}(V/m). It is concluded that an effective reduction of mode hopping is achieved if the laser is operated at a higher bias or if the coupling constant between the two modes is increased.     

Voltage Tunability of Quantum Cascade Lasers

The voltage tunability of three types of quantum cascade laser designs is investigated. The tuning coefficients and tuning ranges of electroluminescence and laser emission from all designs are measured and compared with the calculated results. A reduced tunability was observed in all lasers above threshold. This is attributed to the decrease of resistance across the laser active region (AR) as the photon density increases. A resumed tunability high above threshold occurs in all lasers with anticrossed injector ground and upper laser states. Lasers based on the anticrossed diagonal transition are tunable above threshold, with a tuning range of about 30 ${hbox {cm}}^{-1}$ ($sim$ 3% of the laser emission wavenumber), i.e., a tuning rate of 750 ${hbox {cm}}^{-1} hbox{V}^{-1}cdot hbox{period}^{-1}$ of the AR and the injector.      

Second-harmonic generation in KB5O8· 4H2O from 217.1 to 315.0 nm

Second-harmonic generation (SHG) has been observed in KB5O8ċ 4H2O (KB5) between 217.1 and 315.0 nm by angle tuning in a single crystal using a single cut. A conversion efficiency of 9.2 percent was observed for type I noncritical phase matching at 217.1 nm for a peak power of 15 kW at 434.2 nm. The nonlinear coefficients d31and d32are estimated to be approximately1.1 times 10^{-10}ESU (4.0 times 10^{-25}m/V) and0.08 times 10^{-10}ESU (0.29 times 10^{-25}m/V), respectively.     

Small-signal gain and saturation parameter of a transverse-flow CW oxygen-iodine laser

The small-signal gain and saturation parameter of a transverse-flow CW oxygen-iodine laser have been experimentally obtained for the first time from output power measurements made as a function of the cavity losses without using a CW probe laser. These measurements typically yieldalpha_{0} simeq 0.045m^-1andI_{s} simeq 0.44kW/cm²for a Cl2flow rate of1.4 times 10^{-3}mol/s with an I2flow rate of4 times 10^{-6}mol/s. The dependences of the small-signal gain and saturation parameter have been also found on the Cl2flow rate. These behaviors are qualitatively explained by a simple two-level model.     

New FIR laser lines from optically pumped CH3OH: Measurements and assignments

26 new FIR laser lines have been observed in CO2laser pumped CH3OH, and together with some previously observed lines their wavelengths have been measured with a relative accuracy of3 times 10^{-4}by using the FIR resonator as a scanning Fabry-Perot interferometer. Based on the internal consistency of the data, it is suggested that a number of the lines originate from combination bands involving simultaneous excitation of the CO stretch mode and a different vibrational mode.     

FM eliminated CH4locked frequency stabilization of 3.39 µm He-Ne laser in dual feedback control

The frequency of the 3.39 μm He-Ne laser was locked to the CH4saturated absorption line by means of integral-proportional feedback control, i.e., dual feedback control. The frequency modulation was applied by a vibrating mirror placed outside a laser cavity, obtaining a modulation-free laser beam with a stabilized frequency. The long-term stability achieved under integral feedback control was aboutpm1.1 times 10^{-11}, which was further improved topm1.35 times 10^{-12}under dual feedback control. The Allan variance measured by the photomixing technique was1.77 times 10^{-12}at an averaging time 100 s.     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.