Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

Fluorescence and oscillation characteristics of LiNdP4O12lasers at 1.317 µm

Spectroscopic data and laser oscillation characteristics of the 1.317 μ line in lithium neodymium tetraphosphate (LNP) are reported. A stimulated emission cross section of this transition was spectroscopically determined as7.1 times 10^{-20}cm², which corresponds to 1/4.5 of that at the 1.047-μm transition. Cross-section temperature dependence, laser cavity loss, threshold versus crystal length, and threshold temperature dependence were measured experimentally. Since resonant loss was negligible at the 1.317 μm line, room temperature threshold is lower than that at 1.047-μm in the case of long crystal, and threshold temperature dependence is weaker than that at 1.047 μm. A miniaturized LNP laser, using a graded index fiber as a focusing medium, is also shown. LED pump intensity required to obtain a constant output is compared for 1.047- and 1.317-μm wavelengths. The 1.317-μm line seems to be useful as miniaturized optical sources in optical communication systems, since required intensity is around several W/cm²for the side pump, and the wavelength corresponds to the most transparent band of ultra low-loss optical fibers.     

Atomic laser action in rare gas-SF6mixtures

Pulsed-laser action is described on a number of atomic transitions of ArI, one of which has not been reported previously. The addition of SF6to Ar shifts the laser action in the Ar from the3d'(3/2)min{1}max{0} - 4p'(1/2)_{1}transition to six ArI transitions originating from the three lowest states of the3p^{5}3delectron configuration. Significant enhancement of the intensity is observed over that resulting from a discharge in pure argon. In addition, when a small amount of SF6is added to a He discharge, laser oscillation results from FI at 0.7129 μm. Increased SF6pressure produces an unidentified oscillation at 2.639 μm. These last two transitions have been observed also in He-NF3, He-CF4, and He-PF3mixtures.     

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.     

Saturation and excited-state absorption in neodymium laser glass

The existence of excited-state absorption of 1.06-μ radiation in transitions from the upper laser level upward (⁴F_{3/2}-⁴G_{9/2}) is suggested by the ground-state absorption spectrum of Nd³⁺in soda-lime glass. The strength of this absorption was measured as follows. In the unpumped material the upper laser level was populated at high temperatures (600 and 783°K) by saturating the laser transition with an intense 1.06-μ probe laser beam. The residual absorption was attributed to⁴F_{3/2}-⁴G_{9/2}transitions. The steady-state saturation behavior was calculated on the assumption of rapid spectral cross relaxation, using spectroscopic data appropriate to the elevated temperature. Comparison of theoretical and experimental saturation behavior yielded an effective excited-state absorption cross section equal to one-third the gain cross section of the laser transition. This sizable value may be expected to have a significant adverse effect on laser threshold and efficiency. Also, it provides a mechanism for internal fracture such that the fracture threshold decreases with increasing neodymium concentration and with increasing temperature.     

Near single-line operation of a free-burning CS2/O2/N2O flame laser with a nondispersive optical cavity

The CS2/O2/N2O flame laser has been operated for the first time under conditions in which the spectral output is nearly single line. This transition is theP_{10-9}(17) of CO at 5.4265 μm, the same transition which was observed to oscillate in single-line fashion by Hirose et al. in an electrically initiated CO chemical laser. It is suggested that the unique behavior of this line may be due to its close proximity to aPbranch transition in an adjacent band, namely theP_{9-8}(23) line, such that the gain profiles of the two lines overlap. Calculations suggest that at the conditions of these experiments, the separation of the line centers for this pair is about 0.3 Å or less. TheP_{10-9}(17) transition was also found to be totally absent under certain conditions of high multiline power, particulary at low O2and N2O flows. This may be due to absorption by a high-bandRbranch transition at 5.4266 μm, namely theR_{15-16}(32) line.     

Nonlinear refractive index and "DC" kerr constant of liquid CS2at 10.6 µm

The "dc" Kerr constant of CS2has been measured at 10.6 μm and found to be(2.9 pm 0.8) times 10^{-8}statvolt^-2cm. This yields a value of(2.1 pm 0.7) times 10^{-11}statvolt^-2cm²for the nonlinear refractive index.     

A study of the CW 28-µm water-vapor laser

The low signal gain of a CW water-vapor laser at 28 μm was measured as a function of the discharge current and pressure. Together with the measurement of other quantities such as the axial electric field and the concentration of OH, a partial interpretation of the mechanisms involved in pumping the 28-μm transition was possible. Thermal equilibrium between the ν0,2nu_{2}, and ν3vibrational levels will result in a large absorption at the elevated gas temperatures observed (800-1000 K). The strong dependence of gain on the electron temperature strongly suggests that the vibrational excitation proceeds through electron-impact excitation. Only the electron-impact excitation of H2O is quantitatively capable of overcoming the large thermally induced absorption. Although vibrational-excitation transfer from H2to H2O seems insufficient, by itself, to overcome this absorption, it may provide appreciable additional gain. Pumping of the 28-μm line through electron-ion recombination and by reactions involving OH can be ruled out.     

Assessment of a promethium YAG laser

Radiative transition probabilities have been computed for relaxation of the metastable⁵F1and⁵F_{2} J-manifolds of Pm³⁺YAG. The Pm³⁺YAG system is analyzed for 4-level room-temperaure population inversion leading to 0.92-μ coherent radiation; Pm³⁺YAG is compared to Nd³⁺YAG as a CW laser.     

Iodine-atom laser emission in 2-2-2 trifluoroethyliodide

Lasing at 1.3 μ upon flash photolysis of 2-2-2 tri-fluoroethyliodide, CF3CH2I, has been observed on the²P_{1/2}-²P_{3/2}magnetic dipole allowed transition of atomic iodine. Using an 800-J flash, a maximum peak power output of approximately 108 W for 10-μs duration at half-maximum intensity was obtained with a pressure of 17 torr CF3CH2I.     

Radiative transition probabilities within the 4f3ground configuration of Nd:YAG

The absolute intensities of 20 absorption bands between 2.5 and 0.25 μ have been measured for Nd:YAG at room temperature. These bands are forced electric dipole transitions between the⁴I_{9/2}ground manifold and 34 excitedJmanifolds of the Nd³⁺ion. The transition intensities have been accounted for in terms of three phenomenological parameters with an rms error of 10 percent. The intensity parameters are used to evaluate the line strengths for excited-state absorption from the metastable⁴F_{3/2} Jmanifold. Significant line strengths are predicted at wavelengths near 1.06 and 1.35 μ, suggesting the possibility of radiative depumping of the⁴F_{3/2}upper laser level via the stimulated emission field.     

FM-laser operation of the Nd:YAG laser

The FM-laser or frequency-sweeping mode of laser oscillation has been demonstrated in a Nd :YAG 1.06-μ laser with an intracavity LiNbO3phase modulator. The experimental results are in excellent agreement with the theoretical expressionDelta= (DeltaOmega/Deltanu) (delta/pi)where δ= peak single-pass phase retardation in the modulator,DeltaOmega= axial mode spacing,Deltanu=modulator detuning, and Δ=resulting FM index of the laser output. Modulation indices as large asDelta approx 230rad have been obtained, in which case the instantaneous laser frequency is sweeping over a full spectral range of2Delta cdot f_{m} approx 120GHz (≈ 4 cm^-1) at a repetition frequencyf_{m} approx 260MHz, with a time-bandwidth product per periodapprox 2Delta approx 460. The coherently mode-locked spectral bandwidth thus obtained in the FM-laser case is very much wider than can be achieved in the pulsed mode-locked case with the same Nd:YAG laser. Some possible ways of using this broad-band coherent FM spectrum are suggested.     

Growth, optical properties, and CW laser action of neodymium-doped gadolinium scandium aluminum garnet

Investigation of the congruent melting rare-earth aluminum garnet Gd3Sc2Al3O12(GADSCAG) has been made. The optimum crystal growth parameters for the undoped material are a rotation rate of 20 r/min and a growth rate of 4.6 mm/hr along thelangle111rangledirection. For Nd³⁺-doped crystals the linear growth rate is 2 mm/h. The optical properties of Nd:GADSCAG which are of interest for laser operation were also determined. The stimulated emission cross section of the 1.06-μ transition in Nd³⁺at room temperature is(3.2 pm 0.3) times 10^{-19}cm², the fluorescence lifetime is256 pm 8 mus at an Nd³⁺ion density of 1 atomic percent in the crystal and the integrated peak absorption cross section in the strongest pump band (0.81 μ) is3.8 times 10^{-19}cm², A comparison of the CW laser performance of identical ND:YAG and Nd:GADSCAG rods is presented.     

Observation of saturation peaks in a He-Ne laser by tuned laser differential spectrometry

Light-field-induced saturation peaks in the Ne2p_{4}velocity distribution inside a 0.63-μ He-Ne gas laser are investigated by measuring the resultant frequency-dependent single-pass gain reduction of an additional 1.15-μ He-Ne laser beam.     

Absorption coefficient, transition probability, and collision-broadening frequency of dimethylether at He-Xe laser 3.51-µ wavelength

The pressure dependence of the absorption coefficient of dimethylether (DME) gas at He-Xe laser 3.508-μ wavelength is measured. The experimental results are in fairly good agreement with theory. A collision-broadening frequency of1.6 pm 0.1 times 10^{8}second^-1at 15 torr, 293°K, and a transition lifetime of 2.2 ± 0.2 seconds are obtained. A saturation intensity of about 2 mW/cm²for 1 torr of DME is also estimated.     

Stimulated Raman emission in infrared excited gases

Using the pressure dependence of absorption, absorption coefficients and detunings were measured for CO2pump lines and the strong fat infrared emission in optically pumped D2O. The P (32) CO2line was found to be detuned ∼1.5 GHz from the ν2band transitions6_{6}, 6_{5} rightarrow 5_{5}, 5_{4}. The resulting emission lines at 50.3 μm and 66 μm were found to be detuned from their respective transitions by about the same amount. On the basis of these measurements and gain estimates for the far infrared, the resulting emission lines are identified as stimulated Raman emission.     

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.     

On pressure broadening in a He-Ne laser

Evidence of the combined influence of disaligning and velocity-changing collisions on laser light-induced saturation peaks or holes in the velocity distribution of Ne2p_{4}atoms was obtained by transmitting 1.15-μ light of a short single-mode He-Ne laser through the discharge tube of a long single-mode Michelson interferometer-type He-Ne laser oscillating at 0.63 μ, and by measuring the frequency-dependent single pass gain of the IR light in the discharge of the latter laser. Atoms that have undergone a transition in the linearly polarized red-laser light field are partially aligned. The width of the detected signal depends on the He gas pressure and also on the mutual polarization of the two laser fields. This indicates a smaller velocity broadening for aligned atoms than for collisionally disaligned ones. An evaluation of the data provides information on the correlation of the above-mentioned collision processes and on phase-disturbing and phase-nondisturbing contributions to the line broadening. Estimate values of the integral elastic Ne2p_{4}-He scattering cross section and a cross section for phase shifting collisions are given.     

Emission frequencies of the CF4laser

104 (nu_{2} + nu_{4}) leftarrow nu_{2}hot-band transitions, some of which correspond to known(nu_{2} + nu_{4}) leftarrow nu_{2}CF4laser lines, have been identified and measured in high-resolution Doppler-limited spectra of¹²CF4near 16 μm, obtained with a tunable diode laser. From an analysis of these frequencies, the rotational and tensor splitting constants for the ν2vibrational level have been determined. These constants, together with thenu_{2} + nu_{4}pump-band spectrum previously obtained, allow the laser emission frequencies for any given CO2pump line to be calculated with an accuracy of 0.01-0.003 cm^-1.