A tunable far infrared laser

A continuously tunable far infrared (FIR) laser has been demonstrated; experimental results are presented. A high-pressure (10-12 atm) continuously tunable CO2TE laser is used to pump Raman transitions in CH3F; the generation of continuously tunable radiation in the250-300 mum wavelength range is reported. Accurate frequency and bandwidth measurements have been made and the FIR bandwidth in superradiant emission isapprox4-5GHz. Consequently, the generation of frequency tunable, subnanosecond pulses in the FIR appears feasible. The generation of tunable laser radiation from 150 to 1000 μm by stimulated Raman scattering should be possible using higher pump intensity and/or other gases.     

A continuously tunable sequential Stokes Raman laser

We have obtained continuously tunable coherent radiation in the1-12 mum region via sequential Raman scattering of pulsed-dye-laser radiation in hydrogen. A multiple-pass-cell was used to enhance the Raman gain and produce an overall quantum conversion efficiency of at least 45 percent in the wavelength range from 0.9 to 5μm. At 5 μm, an energy output of 1 mj in a 7 ns pulse at a 10 Hz repetition rate has been obtained. Inherent four-wave mixing initiates the sequential Stokes conversion to the infrared and produces single transverse mode (TEM00) radiation in a 0.2 cm^-1bandwidth. We have developed a nonlinear model of the process that includes the effects of diffraction, four-wave mixing, and temporal pulse shape and gives numerical outputs in agreement with experiment.     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

InGaAsP/InP double-heterostructure lasers: Simple expressions for wave confinement, beamwidth, and threshold current over wide ranges in wavelength (1.1-1.65 µm)

For InGaAsP/InP devices emitting in the wavelength range of1.1-1.65 mum, we present novel and accurate analytical approximations of the crucial parameters in laser design: the radiation confinement factorGamma; the effective refractive index Neff; and the transverse beamwidththeta_{perp}. It is found thatGammais independent of wavelength or step-index difference andtheta_{perp}becomes independent of wavelength as the active layer thicknessesddecreases below 0.15 μm. An explicit analytical expression is derived for the threshold current density Jth. The new linear gain-current relationshipg(cm^-1) = 28.5(J/d) - 50is deduced from a critical assessment of recent experimental data. A theoretical framework is provided to explain experimental observations such as: the Jthversusdcurve is independent of wavelength, the threshold has a negligible variation over the0.1-0.3 mum active-layer thickness range, and the normalized threshold current density (J_{th}/d, ford geq 0.5 mum) varies strongly (3-5 kA/cm²/μm) with cavity length.     

Tunable infrared laser generated via stimulated Raman scattering in Cs vapor

The conversion of a visible dye laser output into tunable infrared (IR) laser radiation via stimulated Raman scattering in Cs vapor was investigated. Small-signal gain, threshold intensity, and conversion efficiency measurements were obtained versus pump laser detuning. Fixed-frequency IR outputs at other wavelengths were also observed. One of these, the7P_{3/2}-5D_{5/2}transition, had a strong lasing component polarized perpendicular to the pump laser polarization. Lasing near the 8523-Å resonance line was attributed to a four-wave parametric interaction as a plausible generation mechanism.     

A Ga0.47In0.53As/InP heterophotodiode with reduced dark current

We have used a Ga0.47In0.53As/InP heterostructure to produce a photodiode (area =3 times 10^{-4}cm²) which shows a saturated dark current of 100 pA at 23°C and 1.7 nA at 50°C. At this dark current, these photodiodes have near-unity quantum efficiency at 1.6 μm and show good photoresponse over the1.0-1.65 mum region of the optical spectrum.     

A universal fiber-optic (UFO) measurement system based on a near-IR fiber Raman laser

A universal fiber-optic measurement system, which is useful for measuring loss and dispersion in the1.06-1.6 mum wavelength region, is described. The source is a silica fiber Raman laser pumped by a mode-locked andQ-switched Nd:YAG laser at 1.06 μm. Subnanosecond multiple-Stokes pulses in the1.1-1.6 mum wavelength region are generated in a low-loss single-mode silica fiber. The use of this near-infrared fiber Raman laser for characterizing various transmission properties of single and multimode test fibers is demonstrated. Loss spectra, intramodal dispersion, and intermodal dispersion data are obtained in the wavelength region of minimum loss and minimum material dispersion for silica fibers.     

High-power difference-frequency generation at 5-11 µm in AgGaS2

Efficient generation of high peak power and high average power difference-frequency radiation, continuously tunable over the range of5-11 mum, has been achieved by mixing the Nd:YAG laser radiation with the output of an infrared dye laser pumped by the same Nd: YAG laser in AgGaS2. Peak pulse powers as high as 180 kW with an average power output of 14 mW were obtained around 6 μm.     

High-power difference-frequency generation at 4.4-5.7 µm in LiIO3

Efficient generation of high-power difference-frequency radiation, continuously tunable over the range of4.4-5.7 mum, has been achieved by mixing the Nd :YAG laser and Nd :YAG pumped infrared dye-laser outputs in LiIO3. Peak pulse powers as high as 550 kW with an average power output of 45 mW were obtained around 4.9μm.     

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.     

Stimulated Raman scattering of picosecond light pulses in hydrogen, deuterium, and methane

Experimental results are presented on stimulated Raman scattering of short pulses of approximately 100 ps duration in H2, D2, and CH4, both in capillary waveguides and in a tight focusing geometry. Experimentally determined thresholds are in good agreement with calculation. Low thresholds (< 20 muJ) are observed in CH4and preliminary results using a mode-locked dye laser as pump indicate a useful source of tunable short pulse radiation in the near infrared.     

Laboratory-simulation experiment for optical communication through low-visibility atmosphere using a diode laser

A laboratory-simulation experiment has been performed to determine the feasibility of exploiting the scattered (in addition to the unscattered) radiation to improve optical communication through low-visibility atmosphere. A multiple field-of-views (FOV) (0.043-0.945degfull angle) optical receiver was designed which utilizes a narrow-band interference filter (Deltalambda sim 15Å) for background-light suppression. The laser transmitter was a CW GaAlAs laser diode (lambda = 0.8486 mum) capable of emitting ∼ 7 mW power output of 15° divergent beam. Both polydisperse (particle diameters,din the range ofsim0.2-5.17 mum) and monodisperse (d sim 0.2 mum andd = 3.01 mum) latex spheres in water were used for simulating haze, fog, etc. Results include signal-to-noise ratio (SNR) and scattered-to-unscattered signal ratio as a function of field-of-view of reception for various optical thickness of the medium. Also, SNR is plotted as a function of optical thickness for various FOV's. The unique feature of this simulation is that it can simulate worst case solar background where the sun enters the FOV of receiver. Finally a new technique of estimating forward-scattering efficiency and root-mean-square forward scatter angle has also been presented with examples.     

Chalcogenide glass fibers for mid-infrared transmission

Chalcogenide glass fibers for mid-infrared transmission have been fabricated in As-S, As-Ge-Se, and Ge-S glass systems using high purity materials. The preparation of unclad, Teflon FEP clad, and chalcogenide glass clad fibers and their transmission loss characteristics are reported. It is found that appropriate glass compositions for drawing low-loss fibers are limited to the narrow ranges in the glass-forming regions. The minimum losses obtained are 35 dB/km at 2.44μm for As40S60unclad fiber, 182 dB/km at 2.12 μm for As38Ge5Se57unclad fiber, and 148 dB/km at 1.68 μm for Ge20S80unclad fiber. It is shown that hydrogen impurity absorptions and short-wavelength weak absorption tails seriously enhance loss in the fibers. It is also suggested that ultralow loss cannot be achieved due to the existence of the weak absorption tail. However, it is expected that the chalcogenide glass fibers can be used in short fiber-length applications such as in the remote monitoring and delivery of CO laser radiation. This is due to their wide operating wavelength ranges of0.9-6mum for As-S,1.3-9mum for As-Ge-Se, and0.8-5mum for Ge-S, in which losses can be reduced to below 1 dB/m.     

Zn-diffused, stripe-geometry, double-heterostructure GaInAsP/InP diode lasers

Deep Zn diffusion from a ZnP2source has been used to fabricate stripe-geometry double-heterostructure GaInAsP/InP diode lasers with emission wavelengths in the1.2-1.3 mum range. These devices exhibit good electrical and optical confinement. For sufficiently narrow stripe widths (<10 mum), emission is usually single mode and linear. CW outputs up to ∼8 mW per facet have been observed. In initial life tests, two Zn-diffused lasers have operated CW at room temperature for over 5000 h without appreciable degradation.     

Tunable near ultraviolet laser system from a frequency doubled alexandrite laser

A laser system which is capable of producing radiation tunable over the region from approximately0.36-0.40 mum is described. The laser produces in excess of 5.0 mJ per pulse in asim0.1 mus pulse length.     

Laser emission in the 83-223 µm region from PH3with laser line assignments

Optical pumping of PH3with a CO2TEA laser has yielded 44 far IR lasing lines in the83-223 mum region. A table of energy levels for the ground state ν2and ν4has been compiled which has permitted the assignment of 16 absorption matches and 40 laser lines. Four of the laser lines appear to have high gain and lase, single pass with CO2mode-locked pumping.     

Dispersion phenomena in hollow alumina waveguides

Significant variations in the attenuation characteristics of hollow alumina waveguides (suitable for use in the manufacture of CO2waveguide lasers) have been measured over the9-11 mum waveband. Existing waveguide transmission theory has been successfully used to explain the qualitative nature of the observed experimental results by taking into account the effects of dispersion due to active lattice absorption bands in alumina. The suprising conclusion of the work is that the major variations in attenuation are a result of a complete change in the guiding nature of the waveguide atsim 10 mum. At this point, the nature of the waveguide changes from being "leaky" at shorter wavelengths to exhibiting attenuated total internal reflection at longer wavelengths. This results in there being a distinct difference in attenuation characteristics for the two main laser emission bands centered at 9.6 and 10.6 μm, respectively.     

Heavy metal halide glass fiber lightwave systems

Heavy metal halide glass fibers have the potential of optical loss between 0.001 and 0.01 dB/km in the2-10 mum region. We have evaluated some of the system aspects of these fibers in order to determine the ultimate performance limits and to assist in defining waveguide design and fiber processing techniques. Extrinsic waveguide-related losses and limitations including microdeformation, optical nonlinearities, dispersion characteristics, and source and detector capabilities become more significant as the intrinsic losses decrease. Two representative halide glass systems are discussed: a heavy metal fluoride operating atsimeq 2 mum and a heavy metal chloride glass atsimeq 6 mum. The results indicate that repeater spacings ≳ 1200 and 3600 km atlsim 1Gbit/s may be possible for chlorides and fluorides, respectively.     

Optical parametric interactions in 3-methyl-4-nitropyridine-1-oxide (POM) single crystals

Various configurations of three-wave parametric interactions are reported at the picosecond time scale, in 3-methyl-4-nitropyridine-1-oxide (POM) single crystals. Wavelength dispersion of phase-matched second-harmonic generation orientation is experimentally established within the0.8-2 mum range using as fundamental input the idler and signal emissions of a picosecond LiIO3parametric emitter. Efficient parametric amplification is demonstrated: at degeneracy a gain of 10³is measured for a pump intensity of 130 MW . cm^-2at 0.53 μm and an input signal of 5 kW . cm^-2at 1.06 μm. Single pass parametric emission is achieved, for the first time in an organic crystal, yielding 200 kW . cm^-2at 1.06 μm, using the frequency-doubled output of a passively mode-locked YAG system. Both dispersion and interaction length dependence of the experimental gain are accounted for.     

High power far-infrared emission from CO2laser-pumped D2S and HDS

Intense superfluorescent emission in the far infrared has been produced for the first time at wavelengths oflambda = 135.7 mum andlambda = 90.1 mum from D2S pumped with a CO2TEA laser. The pump and emission lines have been assigned to molecular transitions in the ν2vibrational mode. An additional strong line atlambda = 183.2 mum has been attributed to the HDS molecule.