Kinetically tailored properties of electron-beam excited XeF(C → A) and XeF(B → X) laser media using an Ar-Kr buffer mixture

Use of a two-component buffer gas comprised of Ar and Kr results in electron-beam excited XeF(C rightarrow A) laser pulse energy and intrinsic efficiency values comparable to those of UV rare gas-halide lasers. Herein we report measurements of transient absorption confirming that the primary effect of a buffer comprised of Ar and Kr is a significantly lower level of ionized and excited species that absorb in the blue-green spectral region. Spectral analysis of a variety of mixtures shows that the Ar-Kr buffer also benefits XeF(C rightarrow A) laser performance due to an increase in gain in the 400-450 nm region caused by the presence of the Kr2F excimer. In addition, a large increase in absorption at ∼ 351 nm, also due to Kr2F, suppresses oscillation on the competitive XeF(B rightarrow X) transition and, for certain conditions, makes efficient simultaneous oscillation of the XeF(B rightarrow X) and XeF(C rightarrow A) laser transitions possible.     

Injection-controlled tuning of an electron-beam excited XeF (C→A) laser

Efficient, ultra-narrow spectral output from an electron-beam excited XeF(C rightarrow A) laser medium has been achieved by injection-controlled tuning. Using a pulsed dye laser as the injection source, amplified output pulses tunable between 435 and 535 nm and having a spectral width of 0.001 nm were obtained. For a 482.5 nm injection wavelength that is well matched to the XeF(C rightarrow A) gain maximum, output energy density and intrinsic efficiency values of approximately 8 J/l and 6 percent were achieved.     

Multijoule performance of the photolytically pumped XeF(C→A) laser

An XeF(C rightarrow A) laser with output up to 5.8 J/pulse has been demonstrated. The photolytic pumping scheme begins withe-beam excitation of xenon to produce Xe*2fluorescence at 172 nm. This VUV radiation is transmitted through an array of CaF2windows into the laser cell, where it photodissociates XeF2to produce primarily XeF(Bfrac{1}{2}). Collisions with N2buffer gas relax the excited states to XeF(Cfrac{3}{2}), which lases on a transition centered at 481 nm and continuously tunable over more than ±35 nm. Typical values of the experimental parameters were as follows. The 420 kV, 1 me-beam source delivered an average current of 10 A/cm²over an aperture 14 × 100 cm for pulse lengths up to 1 μs. Totale-beam energy available was 3.5 kJ, of which 2.4 kJ was deposited in the xenon. The total VUV energy radiated was 720 J, of which 115 J was coupled into the laser cell. This produced 32 J of available XeF* energy, of which up to 18 percent was extracted as laser energy. The total system efficiency was 0.2 percent. Optimized designs should achieve better than 1 percent efficiency.     

Spontaneous emission lifetime of the C → A band of the XeF molecule

The radiative lifetime of the blueC rightarrow Atransition of XeF has been measured to be 93 ± 5 ns by monitoring the exponentially decaying 460 nm emission following rapid electron impact dissociation of XeF2. By varying the XeF2partial pressure, the rate constant for quenching of XeF (C) by XeF2was determined to be(1.8 pm 0.5) cdot 10^{-10}cm³. s^-1. Finally, knowledge of the XeF B and C state lifetimes yields a revised value of 610 ± 60 cm^-1for theBtoCstate energy defect.     

Gain studies of electron beam excited XeF laser mixtures

Detailed optical gain measurements of electron beam pumped Ar/Xe/NF3and Ne/Xe/NF3laser mixtures for theB rightarrow Xand theC rightarrow Atransitions of XeF at 351 and 488 nm are reported using a direct laser probe and two different fluorescence techniques.     

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.     

Excimer laser gain by pulse shape analysis

An analysis of the temporal emission of broadband tunable excimer lasers such as Xe2Cl shows that the laser pulse can be described in terms of aQ-switch model rather than by steady-state considerations. For electron-beam (e-beam) pumped rare gas halide mixtures, transient absorption of the majority gas-usually argon-acts as a chemicalQ-switch. The ring-up time of the delayed laser pulse may be conveniently used for optimization of the laser gas mixture.     

Simultaneous UV/visible laser oscillation on the B → X and C → A XeF excimer transitions

Simultaneous laser oscillation from the 351 nm XeF(B rightarrow X) transition and the broad-band XeF (C rihgtarrow A) transition centered near 475 nm has been demonstrated using intense, short-pulse electron-beam excitation of high-pressure gas mixtures. Analysis of the causes of transient absorption suggests that it may be possible to obtain efficient UV/visible laser oscillation from each of the XeF excimer transitions excited in the same medium.     

Pressure dependency of the NF3-H2transverse-discharge pulse-initiated HF chemical laser

The pressure dependency of the performance of the NF3-H2chemical-laser system has been evaluated. The laser energies at various chemical compositions and initiation discharge energies are presented as functions of Pressure. Overall efficiency, laser pulsewidth, and effects of additive gases are also presented. For all compositions the laser energy maximized at a pressureP_{max}which was observed to be dependent on the pulsewidth of the initiation discharge. At pressures belowP_{max}the laser energy was proportional to the partial pressure of NF3or H2. No change in slope of the laser energy versus pressure curve was observed in going from low-pressure nonexploding regimes to high-pressure exploding regimes, implying that the reactions causing the explosion did not contribute to the lasing. Lasing usually occurred in two peaks, the first containingHF(V=2) rightarrow HF(V=1)lines and the second containingHF(V=3) rightarrow HF(V=2)andHF(V=1) rightarrow HF(V=0)lines. These data indicate that lasing is due to the reaction sequence: 1)NF_{3} + e^{-} = cdot NF_{2} + Fcdot + e^{-}; 2) Fcdot + H_{2} = HF+(V) + Hcdot; and 3)HF+(V) + hnu = HF+(V-1) + 2hnu.     

Photoexcitation and photodissociation lasers--Part I: Nitric oxide laser emissions resulting from C(2π) → A(2Σ+) and D(2Σ+) → A(2Σ+) transitions

-1-μm laser emission was detected when NO or NO2was flash-photolyzed, with or without dilution, in the vacuum UV above 165 nm. The emission was identified as theC(²π)rightarrow A(²Sigma+)     

Effects of focusing on third-order nonlinear processes in isotropic media

A theoretical analysis of third-order nonlinear interactions of focused laser beams is performed for the processesomega_{1} + omega_{2} + omega_{3} rightarrow omega_{4}, omega_{1} + omega_{2} - omega_{3} rightarrow omega_{4}, andomega_{1} - omega_{2} - omega_{3} rightarrow omega_{4}. The total power and far-field beam profile of the generated radiation is related to the total powers of the fundamental beams, to the tightness and location of the focus, and to the value of the difference between the wave vectors of the generated radiation and driving polarization. The optimum degree of wave-vector mismatch as a function of tightness and location of focus is determined for each of the three processes. The processomega_{1} + omega_{2} - omega_{3} rightarrow omega_{4}is found to be unique in that it is always optimized by focusing as tightly as possible. Experimental results, which verify the theory for the processesomega_{1} + omega_{2} + omega_{3} rightarrow omega_{4}andomega_{1} + omega_{2} - omega_{3} rightarrow omega_{4}, are presented.     

Blue-green atomic photodissociation lasers in group IIb: Zn, Cd, and Hg

Four new laser lines, two in atomic Zn at 481.1 and 472.2 nm and two in atomic Cd at 508.6 and 480.0 nm, are reported. The analog laser lines in Hg at 546.1 and 435.8 nm are studied further. These lines correspond to the transitionsn^{3}S_{1} rightarrow (n-1)^{3}P_{2}andn^{3}S_{1} rightarrow (n-1)^{3}P_{1}of the triplet system of the metals. The medium in the three cases is MI2(M= Zn, Cd, or Hg) vapor at about 1 mbar and the pump is a KrF laser at 248 nm. The blue-green superfluorescent pulse power is in the kilowatt range, with a pulse duration of about 1 ns. The pumping process must involve more than one photon. A sequential three-photon pumping process is proposed for the three cases.     

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO2laser, area ∼ 8 cm², initiates an explosive reaction between SF6and SiH4. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)sim 1.5 /mus] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <p(SiH4)/p(SF6) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S2(B^{3}Sigma-_{u} rightarrow X^{3}Sigma-_{g}). Transitionsupsilon' (0-4) rightarrow upsilon" (2-15)were recorded. In the³Sigma-_{u}state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH4)/(SF6) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH4)/(SF6) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF6(0.2-0.5 percent of initial amount) is enriched in³⁴SF6; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO2laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.     

Asymptotic performance and complexity of a coding scheme for memoryless channels

The purpose of this paper is to show that decoding complexity need not grow exponentially with the code block length at rates close to channel capacity and also to show the expediency of the approach of imbedding codes in each other. It is demonstrated that it is possible to communicate over a memoryless channel of capacityCat any rateR < Cwith a probability of error of less than2^{-E(R)nu}, E(R) > 0, per block of a length approximately proportional tonu^{2}and with a computational decoding complexity per digit which is asymptotically proportional tonu^{alpha}whennuis large,nu^{alpha}being finite forR < C.(alpha rightarrow mbox{as} R rightarrow C, alpha rightarrow 2 mbox{as} R rightarrow 0).     

Visible-light laser oscillation in indirect-bandgap AlxGa1-xAs (77 K)

Laser oscillation was accomplished at 77 K for the first time in indirect-gap AlxGa1-xAs (x = 0.46) incorporated with nitrogen atoms by ion implantation. Pumping was made by a coumarin 102 dye laser (480 nm). Threshold pumping power was6 times 10^{4}W/cm²in nitrogen-doped AlxGa1-xAs (x = 0.46), and laser oscillation was not observed in undoped samples. Nitrogen atoms, introduced into the undoped AlxGa1-xAs active layer, were found to work as isoelectronic traps and to be responsible for stimulated emission and laser oscillation.     

On mean-square aliasing error in the cardinal series expansion of random processes (Corresp.)

An upper bound is derived for the mean-square error involved when a non-band-limited, wide-sense stationary random processx(t)(possessing an integrable power spectral density) is approximated by a cardinal series expansion of the formsum^{infty}_{-infty}x(n/2W)sinc2W(t-n/2W), a sampling expansion based on the choice of some nominal bandwidthW > 0. It is proved thatlim_{N rightarrow infty} E {|x(t) - x_{N}(t)|^{2}} leq frac{2}{pi}int_{| omega | > 2 pi W}S_{x}( omega) d omega,wherex_{N}(t) = sum_{-N}^{N}x(n/2W)sinc2W(t-n/2W), andS_{x}(omega)is the power spectral density forx(t). Further, the constant2/ piis shown to be the best possible one if a bound of this type (involving the power contained in the frequency region lying outside the arbitrarily chosen band) is to hold uniformly int. Possible reductions of the multiplicative constant as a function oftare also discussed, and a formula is given for the optimal value of this constant.     

On the capacity of certain additive non-Gaussian channels

A model of an additive non-Gaussian noise channel with generalized average input energy constraint is considered. The asymptotic channel capacityC_{zeta}(S), for large signal-to-noise ratioS, is found under certain conditions on the entropyH_{ tilde{ zeta}}( zeta)of the measure induced in function space by the noise processzeta, relative to the measure induced bytilde{zeta}, where is a Gaussian process with the same covariance as that ofzeta. IfH_{ tilde{zeta}}( zeta) < inftyand the channel input signal is of dimensionM< infty, thenC_{ zeta}(S)= frac{1}{2}M ln(1 + S/M) + Q_{zeta}( M ) + {o}(1), where0 leq Q_{ zeta}( M ) leq H_{ tilde{ zeta}}( zeta). If the channel input signal is of infinite dimension andH_{ tilde{ zeta}}( zeta) rightarrow 0forS rightarrow infty, thenC_{ zeta}(S) = frac{1}{2}S+{o}(1).     

5B6--Magnetic depolarization of a vapor and polarization of a Zeeman laser

Lamb's self-consistent theory and rate equation approximation are used to determine the polarization of a monomode gas laser subjected to a magnetic field. The excited vapor is represented by a four level system; its motion is described in a Bloch analog form, using Fano formalism. In the stimulated emission (absorption) process, atoms and light are shown to exchange energy and angular momentum. This optically induced angular state of the vapor can be destroyed by the applied magnetic field, lowering the optical saturation. When the Zeeman sublevels no longer overlap, the laser tends to oscillate with a transverse polarization of minimized saturation. Experimental evidence of these changes of polarization is given with an internal plane mirror laser on the 1.52 μm (J= 1 rightarrow J= 0) line of Ne²⁰. The observed data are pressure sensitive and give some information on the lifetime and disorienting cross section of the2s_{2}level of Ne.     

On Sequential M-ary Orthogonal Modulation

This paper examines aM-ary sequential orthogonal (OM) communications system. A similarM-ary sequential pulse-amplitude modulation (PAM) problem was previously considered by Hecht and Schwartz [10], and by the author. We examine the following problem: given that one of them signals is repetitively sent over an additive Gaussian channel successive intervals of time, determine the optimum sequential procedure to follow at the receiver to pick the correct signal with a probability of wrong selection no greater thanvarepsilon. The optimum procedure is defined to be one that minimizes the expected number of transmissions (sample size) before a decision is reached. This paper extends the works of Viterbi [1] and Kramer [2] who proposed two ad hoc test procedures for this OM problem. From the results of Simons [3] and Hoeffding [4], a lower bound on the expected sample size is found for any sequential test procedure for the proposed OM system. Both the maximum a posteriori probability (MAP) and the simultaneous tests are shown to achieve this lower bound in the limit asepsilon rightarrow 0. Viterbi [1] computed the expected sample size for the MAP test procedure whenM = 2. For the case whereM geq 3, this paper derives approximations of the expected sample sizes for both the MAP and the simultaneous test procedures whenvarepsilon rightarrow 0. Computer simulation shows that the derived expected sample size gives accurate estimation of the actual sample size whenM epsilon < 0.1. These sequential procedures offer 1 to 2 dB more power saving than the test procedures of Viterbi [1] and Kramer [2] ifepsilon > 10^{-7}.     

Capacity theorems for the relay channel

A relay channel consists of an inputx_{l}, a relay outputy_{1}, a channel outputy, and a relay senderx_{2}(whose transmission is allowed to depend on the past symbolsy_{1}. The dependence of the received symbols upon the inputs is given byp(y,y_{1}|x_{1},x_{2}). The channel is assumed to be memoryless. In this paper the following capacity theorems are proved. 1)Ifyis a degraded form ofy_{1}, thenC : = : max !_{p(x_{1},x_{2})} min ,{I(X_{1},X_{2};Y), I(X_{1}; Y_{1}|X_{2})}. 2)Ify_{1}is a degraded form ofy, thenC : = : max !_{p(x_{1})} max_{x_{2}} I(X_{1};Y|x_{2}). 3)Ifp(y,y_{1}|x_{1},x_{2})is an arbitrary relay channel with feedback from(y,y_{1})to bothx_{1} and x_{2}, thenC: = : max_{p(x_{1},x_{2})} min ,{I(X_{1},X_{2};Y),I ,(X_{1};Y,Y_{1}|X_{2})}. 4)For a general relay channel,C : leq : max_{p(x_{1},x_{2})} min ,{I ,(X_{1}, X_{2};Y),I(X_{1};Y,Y_{1}|X_{2}). Superposition block Markov encoding is used to show achievability ofC, and converses are established. The capacities of the Gaussian relay channel and certain discrete relay channels are evaluated. Finally, an achievable lower bound to the capacity of the general relay channel is established.