Visible spectrum absorption cross section of Cr3+in the2E state of pink ruby

Absorption cross sections of Cr³⁺in the metastable²E states of pink ruby (0.05 percent Cr2O3) have been measured forE perp Cbetween 4000 Å and 7000 Å at 297°K and 115°K. Both 0 degree and 90 degree ruby rods were measured in a flash absorption apparatus having a spectral resolution of 50 Å. The broad excited state absorption bands centered at 5600 Å appear to correlate with the positions of²T1(³T1) level, while the narrow band centered at 4540 Å correlates well with the predicted²A2(¹E) level. The effect of excited state absorption on laser operation is discussed in this paper.     

Experimental upper limit to the time of nonradiative relaxation between4T2and2Estates in ruby

An upper limit of5 times 10^{-10}seconds has been determined to the time of nonradiative relaxation between⁴T2and²Estates of ruby. The value was inferred from slope comparison of the experimental and theoretical rise curves ofR-line fluorescence.     

Difficulties associated with stimulated emission in a bismuth--Neon discharge

The population of the metastable6p^{3}²D_{3/2}^{0}bismuth level has been determined by time-resolved absorption spectroscopy at times in the range1-10 mus after the start of the discharge in a bismuth vaporneon mixture, for a neon pressure of 13 mbar and a specific discharge energy of 1 mJ . cm^-3. An exponential decay of the population is observed and lifetimes of 3.0, 4.5, and 5.3 μs are obtained at temperatures of 830, 790, and 755°C, respectively. Measurements of the nine strongest fluorescent transitions to the metastable level confirm that a population of the order of 10¹³cm^-3is established in the early stage of the discharge and exceeds by three orders of magnitude the peak population of the bismuth resonance level6p^{2} 7s^{4}P_{1/2}. It is concluded that the absence of laser emission at 472.2 nm in the system studied is due to excessive population of the metastable level6p^{3}²D_{3/2}^{0}during the early stage of the discharge. It is proposed that this high population arises from the dissociation of Bi2dimers by electron impact and that the loss of electron energy to dissociation and to excitation of the vibrational and rotational levels of the dimers is responsible for the very low population achieved in the6p^{2} 7s⁴p_{1/2}level.     

Far-infrared lasing in ruby

The feasibility of submillimeter wavelength (870 GHz) lasing in Al2O3is discussed. The proposed scheme is based on the pumping of the2bar{A}level of ruby via a ruby laser operating on its R2line. Lasing is expected on the2bar{A} rightarrow bar{E}transition of the split²Elevel.     

Quantum counter for far-infrared radiation

Two types of three-level quantum counters, based on the Bloembergen and the inverted scheme, are discussed with respect to their application for up-conversion of far-infrared (FIR) radiation to visible light. It is reported on a tunable quantum counter of the inverted type: in optically pumped ruby, photons at 29 cm^-1are absorbed by the transitionbar{E}(²E) →2bar{A}(²E) and lead to an increased R2- fluorescence radiation. By Zeeman splitting of the²Elevels, tuning of the detected frequency from 29 cm^-1to higher and lower frequencies is possible (with 1 cm^-1/Tesla). Using the 891-GHz radiation of an HCN laser we determine for the quantum counter a power conversion efficiency of the order of 10^-6and a frequency bandwidth of 360 MHz. We find the interesting result that phonon bottleneck due to resonant trapping of 29 cm^-1phonons can lead to an increase of the quantum efficiency by orders of magnitude.     

Electronic- and phonon-terminated laser emission from Ho3+in BaY2F8

Infrared spontaneous and stimulated emission from Ho³⁺in BaY2F8is reported. In addition to the familiar⁵I_{7} rightarrow⁵I8transition at 2 μ,⁵F_{5} rightarrow⁵I5emission at 2.4μ and⁵I_{6} rightarrow⁵I7emission at 2.9μ are discussed. There are several unusual features of the 2-μ laser emission. At room temperature, phonon-terminated laser emission is observed at 2.171 μ. At 77 K a complex CW laser output is observed in a wavelength interval lying on the shoulder of a fluorescence line. The complex output is attributed to oscillation in transverse modes of the resonator. Oscillation is not observed in the strongest emission line, despite a large terminal state splitting of 310 cm^-1. These results are explained on the basis of a theory developed earlier for transition metal ion lasers. The validity of the model is supported by demonstrating the tunability through loss modulation predicted by theory. The observation of these effects is made possible by the very low internal scattering loss in the crystals. The⁵F_{5} rightarrow⁵I5laser lines near 2.4 μ represent relatively low gain transitions with pulse durations limited by accumulation in a longer lived terminal state. The dynamics of laser emission indicate the possible absence of thermal equilibrium in the excited state. For the 2.9-μ transition the bottleneck posed by a longer lived terminal state may be eliminated by the addition of Eu³⁺or Pr³⁺, but laser emission could not be obtained.     

Note on near-infrared operating characteristics of the mercury ion laser

Additional excitation mechanisms, apparently overlooked in [1], but previously reported to be responsible for the selective excitation of the Hg II7p²P_{3/2}upper state of the 6150-Å laser line in mercury-helium lasers, are referenced and discussed. It is pointed out that asymmetric charge transfer and Penning-type ionization reactions can also account for the selective excitation of the Hg II7p²P_{3/2}state as well as direct electron impact excitation from the Hg I ground state.     

Induced-emission cross sections in neodymium laser glasses

A method for calculating induced-emission cross sections in neodymium laser glasses, based on simple absorbance measurements, has been demonstrated. Absorption and emission transition probabilities of four silicate-base neodymium laser glasses have been characterized in terms of the Judd-Ofelt (JO) model of crystal-field-induced electric-dipole transitions. Absolute absorption intensities in 3669A, ED-2, LSG-91H, and S33 glasses were measured and used to determine three phenomenological intensity parameters for each glass which accounted for the measured intensities to within 5 percent (rms). Emission intensities between the metastable⁴F_{3/2}and terminal⁴I Jlevels were then calculated for each glass, together with the⁴F_{3/2}radiative lifetime, fluorescence branching ratios, and radiative quantum efficiency. Calculated values of these quantities for 3669A and S33 glasses are compared with measured values appearing in the literature. Using experimental line shapes for the⁴F_{3/2} rightarrow⁴I_{11/2}transitions at 1.06μ, induced-emission cross sections for these glasses were calculated.     

The influence of the optical stark shift on the two-photon excitation of the molecular hydrogen E, F1Σ+gstate

The first demonstration of the use of the optical Stark effect to secure resonance for a two-quantum process in the ultraviolet is reported. Following excitation of molecular hydrogen by 193 nm radiation, intense stimulated emission on both the Lyman and Werner bands is observed. The Werner (2-5)Q(1)transition at 117.5 nm exhibits a conversion efficiency of ∼0.2 percent. It is found that electron collisions with excited molecules provide the most probable mechanism for excitation of theC^{1}Pi_{u}levels with a rate constant estimated to bek_{e} sim 7 times 10^{-6}cm³/s.     

Short unit-memory byte-oriented binary convolutional codes having maximal free distance (Corresp.)

It is shown that(n_{o},k_{o})convolutional codes with unit memory always achieve the largest free distance among all codes of the same ratek_{o}/n_{o}and same number2^{Mk_{o}of encoder states, whereMis the encoder memory. A unit-memory code with maximal free distance is given at each place where this free distance exceeds that of the best code withk_{o}andn_{o}relatively prime, for allMk_{o} leq 6and forR = l/2, 1/3, 1/4, 2/3. It is shown that the unit-memory codes are byte oriented in such a way as to be attractive for use in concatenated coding systems.     

Ground state ESR measurements of ruby during laser action

The use of electron spin resonance for measurement of the ground state population (GSP) of Cr³⁺in a ruby rod during optical pumping is described. For a 7.7 cm long rod at 90°K and a mirror reflectance productR_{1}R_{2} > 0.08, the GSP at laser threshold was found to be 55 ± 5 percent in good agreement with theory and previous optical measurements. At room temperature, a limiting value of approximately 30 percent for the GSP was approached at high pump energies. Observations of the relativeR_{1}(3/2), R_{1}(1/2)thresholds at 90°K as a function of mirror reflectivity indicated that the²E excited state absorption cross section at the laser wavelength is less than5 times 10^{-21}cm². Detection of total internal reflection oscillations in cooled ruby lasers from ESR observations is also described.     

An estimate of the variation of a band-limited process

Upper and lower bounds are established for the mean-square variation of a stationary processX(t)whose power spectrum is bounded byomega_{c}, in terms of its average powerP_{0}and the average powerP_{1}of its derivative. It is shown thatleft( frac{2}{pi} right)^{2} P_{1} tau^{2} leq E {|X(t+tau )-X(t)|^{2}} leq P_{1} tau^{2} leq omega_{c}^{2}P_{0}tau^{2}where the upper bounds are valid for anytauand the lower bound fortau < pi / omega_{c}. These estimates are applied to the mean-square variation of the envelope of a quasi-monochromatic process.     

A simplified method for determination of the induced-emission cross section of neodymium doped glasses

Induced-emission cross sections for the⁴F_{3/2}rightarrow⁴I_{11/2}and⁴F_{3/2} rightarrow⁴I_{13/2}transitions of Nd³⁺in glasses can be determined with a satisfactory precision by a simplified method, based on the Judd-Ofelt theory, from the measurement of the integrated absorbance of the 750-nm band and of the effective linewidth of the fluorescence band.     

Broad-band gain in optically pumped S2

TheB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g}system of S2has many of the characteristics required of a tunable laser: a broad fluorescence spectrum and a Franck-Condon shift between theBandXstates. In this paper, theupsilon' = 4level of theB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g}system of S2is pumped by the absorption of the 308 nm XeCl excimer radiation. Rare gas-S2collisions redistribute the population among theupsilon'levels, and the subsequent emission contains intense bands throughout much of the neat UV and visible region of the spectrum. Broad-band gain is reported on two blue-green bands (2,17) and (3,18) populated by rare gas relaxation. Absorption is reported on the near UV bandsupsilon' = 0 rightarrow upsilon" rightarrow 6,7,8where the lower levels are filled by the transient population cascading down theXstate manifold. A fluorescence study is presented which examines the potential advantages and limitations of this system.     

New room temperature CW laser transitions in YAlG:Nd

Using a dispersive prism, the room temperature YAlG : Nd laser has been made to oscillate CW on a total of seven⁴F_{3/2} rightarrow⁴I_{11/2}transitions ranging in wavelength from 1.0519 μm to 1.1226 μm and a single⁴F_{3/2} rightarrow⁴I_{13/2}transition at 1.319 μm.     

Effect of band shapes on carrier distribution at high temperature

This paper indicates both experimentally and graphically how the distribution of carriers changes above a critical temperature Tc, which is related to the density of states ρ in the band tails. Thus,kT_{c} = E_{0}, whererho = rho_{0} exp (DeltaE/E_{0}). The emission spectrum shifts with changing excitation (band filling) at lower temperatures (T < T_{c}) and stays at a constant value independent of excitation at higher temperatures (T > T_{c}). Experimental results of photoluminescence studies with GaAs crystals show that the quantity E0, obtained from the dependence of the peak energy upon the degree of excitation is the same as the quantitykT'_{c}, obtained from the dependence of the peak energy upon temperature. This is in agreements with the analysis. For lasing diodes, the threshold will increase steeply with increasing temperature above Tc, if the excited carriers are located mostly in the exponential states of the band tail.E_{0} = (10 sim 20)meV and Tc= (120 sim 250degK) are typical for crystals of heavily compensated GaAs.     

The effects of laser pulsewidth and molecular lifetime on the experimental determination of one-photon and two-photon excitation spectra

We demonstrate that it is important to know both the observed fluorescence lifetime (τ) and the excitation pulsewidth (Gamma) in order to determine the effect of changes inGammaon the measurement of one- and two-photon excitation spectra. IfGamma/tauis greater than 3, it is sufficient to measureI_{F}/I_{L}^{n}orI_{F}/N_{L}^{n}forn-photon excitation spectra (n = 1and 2), where IFis the peak fluorescence intensity, ILis the peak intensity of the excitation pulse, and NLis the total number of photons per pulse. WhenGamma/tauis less than 3, however, the pulsewidth dependence ofI_{F}/I_{L}^{n}orI_{F}/N_{L}^{n}should be treated by using the pulse-width correction functions defined in this paper. A simple method to estimate observed fluorescence lifetimes is also described where one needs only to measureGammaand the time interval between the peak intensity of an excitation pulse with a Gaussian or near Gaussian temporal profile and the peak fluorescence intensity. Tables of correction factors are provided which can be used to generate accurate values of these functions for arbitrary ratios ofGamma/tauusing interpolation techniques. The treatment of the effect of non-Gaussian excitation pulses on the measurement of one- and two-photon excitation spectra is discussed.     

Behavior of first- and second-positive emission in the N2/SF6laser

Experimental results for the behavior of the emission of the discharge-excited first- and second-positive bands of N2and flow-tube measurements for the deactivation of theA^{3}Sigmamin{u}max{+}andB^{3}Pi_{g}levels of N2by SF6are presented. The results of both these experiments are used to explain the operating mechanisms of the N2/SF6laser.     

An experimental upper limit to the nonradiative relaxation time between the4T2and2T1states of ruby

An upper limit of 4 ns has been determined for the nonradiative relaxation times between the⁴T2and²T1states of ruby. This value was deduced from the rate of rise of the red²T1fluorescences from ruby crystals pumped by the second harmonic of a 1.06-μ laser.