Frequency offset locking of AlGaAs semiconductor lasers

Frequency offset locking was proposed as a reliable electrical negative feedback technique for tracking and sweeping of a semiconductor laser frequency. A frequency stabilized laser was used as a master laser, whose residual frequency fluctuations were 140 (kHz) at the integration time (τ) of 100 msleq tau leq 100s. A digital phase comparator of a large dynamic range of2pi times 2^{11}(rad) was employed in the feedback loop to reduce the phase fluctuations of the beat signal between the master and slave lasers. Performances of frequency tracking and sweeping of the slave laser were quantitatively evaluated, and the results are: residual frequency fluctuations of the beat signal were reduced as low as 11 (Hz) attau = 100s, which meant that the residual frequency fluctuations of the slave laser were almost equal to those of the master laser, i.e., the slave laser frequency tracked accurately to the master laser frequency. Both the capture range and lock range of the beat frequency were 1.22 GHz. Frequency tunable range of the slave laser was 36.6 GHz under the condition of frequency offset locking, in which the slave laser frequency fluctuations were maintained as low as the one given above.     

4C1--Nonlinear effects in an NMR experiment

We have studied multiple-photon transition in a low-field nuclear magnetic resonance (NMR) experiment. It is well known that a saturation of the NMR occurs when using a sufficiently high alternating magnetic field which induces multiple-photons transitions. We show theoretically that these multiphoton transitions are accompanied by an harmonic generation. For an-photon transition, this generation occurs mostly on thenth and (n pm 1)th harmonic. For an irradiating field2H_{1} cos omegat of pulsation ω such thatnomega= gamma H_{0}=omega_{0}where H0is the steady field and θ is the angle between H0and H1the amplitudesSmin{n}max{n}andSmin{n}max{n+1}of these components are given bySmin{n}max{n} = f_{n}(theta) (H_{1})^{n},Smin{n}max{n+1} = cos theta f_{n}(theta) (H_{1})^{n+1}. We have verified these equations in a low-field NMR experiment (H_{0} = 0.7Gs/s), polarizing first a flowing liquid in a high magnetic field. The liquid then flows in a modified Bloch spectrometer. The receiving coil is still perpendicular to the steady field but it is possible to adjust the angle θ between the axis of the emission coils (H1field) and the steady-field H0. Using a synchronous detection at the output of the receiving coil on the pulsation(n - 1) omega, nomega, or(n + 1) omega, we have directly detected multiphoton transistions. The above equations have been verified forn = 2ton = 5. There are slight discrepancies at high excitation which may be explained taking into account a large saturation.     

Pulsed discharge initiated chemical lasers -- Part II: HF laser emission from NF3and N2F4systems

A transverse, multiple-arc pulsed discharge has produced laser emission in NF3and N2F4mixed with H2, CH4, C2H6, HCl, HBr, and natural gas.P_{10}, P_{21}, P_{32}, and P43HF transitions were observed. The peak powers measured ranged from ∼8.5 to 25 kW with typical pulsewidths ofsim0.3 mus.     

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 topm 1.35 times 10^{-12}under dual feedback control. The Allan variance measured by the photomixing technique was1.77 times 10^{-12}at an average time 100 s.     

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.     

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.     

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.     

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.     

Very high resolution CARS spectroscopy in a molecular beam

Signal-to-noise calculations are presented for coherent anti-Stokes Raman spectroscopy (CARS) in a molecular beam. Using high peak power pulsed lasers and a high density molecular beam,S/Nfor the H2Q(1) line is expected to besim 10^{4}, with a Doppler width reduction from 1100 to 20 MHz.     

Pure rotational HF laser oscillations from exploding-wire laser

We report 19 pure rotational HF laser transitions, 5 of which are new, from exploding-wire, metal atom oxidation lasers. The laser operates at F2pressures as high as 500 torr and at a gain length of only 0.24 m. The laser pulses are1-4 mus duration and deliversim10^{-4}J/pulse.     

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.     

Hypothesis testing with multiterminal data compression

The multiterminal hypothesis testingH: XYagainstH̄: X̄Ȳis considered whereX^{n} (X̄^{n})andY^{n} (Ȳ^{n})are separately encoded at ratesR_{1}andR_{2}, respectively. The problem is to determine the minimumbeta_{n}of the second kind of error probability, under the condition that the first kind of error probabilityalpha_{n} leq epsilonfor a prescribed0 < epsilon < 1. A good lower boundtheta_{L}(R_{1}, R_{2})on the power exponenttheta (R_{1}, R_{2},epsilon)= lim inf_{n rightarrow infty}(-1/n log beta_{n})is given and several interesting properties are revealed. The lower bound is tighter than that of Ahlswede and Csiszár. Furthermore, in the special case of testing against independence, this bound turns out to coincide with that given by them. The main arguments are devoted to the special case withR_{2} = inftycorresponding to full side information forY^{n}(Ȳ^{n}). In particular, the compact solution is established to the complete data compression cases, which are useful in statistics from the practical point of view.     

Frequencies and Wavelengths From a New Far-Infrared Lasing Gas: $^{13}$ CHD$_{2}$ OH

The first laser action produced by the partially deuterated isotopic form of methanol, $^{13}$ CHD$_{2}$ OH, has been observed. With this laser medium, eight far-infrared laser emissions were discovered having wavelengths ranging from 33.8 to 80.9 $mu$m. A three-laser heterodyne system was used to measure the frequencies for these newly discovered laser lines and are reported with fractional uncertainties of ${pm}3 times 10^{-7}$. The offset frequency of the CO$_{2}$ pump laser was measured with respect to its center frequency for each $^{13}$CHD$_{2}$OH laser emission.      

4C6--Concentration and temperature dependences of spin-lattice relaxation times in ruby at helium temperatures: Relaxation in a zero magnetic field

The relaxation times T1for the grouud state levels in ruby were measured in the temperature range 4.2 to 1.6°K for the various concentrations of theCr^{3+}ionsffrom 0.05 to 0.7 percent. The dependenceT_{1}(f)of the formT_{1}^{-1}(f) = T_{1}^{-1}(0) + T_{1}^{-1}(I)f ^{n}withn simeq 2has been obtained for the different transitions. The measurements of relaxation times forpm frac{1}{2} leftrightarrow pm frac{3}{2}transition at zero magnetic field were especially aimed at establishing a form of dependenceT_{1}(f)because of the absence of the cross relaxation effects in this case. The normal temperature dependenceT_{1} propto T_{1}^{-1}has been obtained at all concentrations in comparison with anomalous dependences observed at high concentrations by some researchers.     

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.      

A nonparametric estimation of the entropy for absolutely continuous distributions (Corresp.)

LetF(x)be an absolutely continuous distribution having a density functionf(x)with respect to the Lebesgue measure. The Shannon entropy is defined asH(f) = -int f(x) ln f(x) dx. In this correspondence we propose, based on a random sampleX_{1}, cdots , X_{n}generated fromF, a nonparametric estimate ofH(f)given byhat{H}(f) = -(l/n) sum_{i = 1}^{n} In hat{f}(x), wherehat{f}(x)is the kernel estimate offdue to Rosenblatt and Parzen. Regularity conditions are obtained under which the first and second mean consistencies ofhat{H}(f)are established. These conditions are mild and easily satisfied. Examples, such as Gamma, Weibull, and normal distributions, are considered.     

Second-harmonic generation in CDA and CD*A

Efficient second-harmonic generation at 5321 Å has been achieved in CDA and CD*A. At 50-MW pump power, loss-free peak power-conversion efficiency of 57 percent and 45 percent were obtained in CDA and CD*A, respectively, The nonlinear optical constant, corrected for multimode fluctuations, was found to bed_{36}(CDA) = d_{36}(CD*A) = (0.96 pm 0.11) times 10^{-9}ESU, which corresponds tod_{36}(CDA) = d_{36}(CD*A) = 0.92 times d_{36}(KDP). In addition, the temperature variation of the birefringenced(n_{2}^{e} - n_{1}^{o})/dThas been measured to be(8.0 pm 0.2) times 10^{-6}degC^-1for CDA and(7.8 pm 0.2) times 10^{-6}degC^-1for CD*A.     

Dispersion of the group velocity refractive index in GaAs double heterostructure lasers

The dispersion of the refractive index corresponding to the group velocityn*_{1}has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence ofn*_{1}on wavelength contains an approximately constant term due to the refractive index n1and a strongly dispersive component due to-lambda (partialn_{1}/ partiallambda). For a given spectral bandwidth, the dispersion ofn*_{1}causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10^-13s should be obtainable from such mode locked semiconductor lasers.     

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.     

On single-sample robust detection of known signals with additive unknown-mean amplitude-bounded random interference--II: The randomized decision rule solution (Corresp.)

A randomized decision rule is derived and proved to be the saddlepoint solution of the robust detection problem for known signals in independent unknown-mean amplitude-bounded noise. The saddlepoint solutionphi^{0}uses an equaUy likely mixed strategy to chose one ofNBayesian single-threshold decision rulesphi_{i}^{0}, i = 1,cdots , Nhaving been obtained previously by the author. These decision rules are also all optimal against the maximin (least-favorable) nonrandomized noise probability densityf_{0}, wheref_{0}is a picket fence function withNpickets on its domain. Thee pair(phi^{0}, f_{0})is shown to satisfy the saddlepoint condition for probability of error, i.e.,P_{e}(phi^{0} , f) leq P_{e}(phi^{0} , f_{0}) leq P_{e}(phi, f_{0})holds for allfandphi. The decision rulephi^{0}is also shown to be an eqoaliir rule, i.e.,P_{e}(phi^{0}, f ) = P_{e}(phi^{0},f_{0}), for allf, with4^{-1} leq P_{e}(phi^{0},f_{0})=2^{-1}(1-N^{-1})leq2^{-1} , N geq 2. Thus nature can force the communicator to use an {em optimal} randomized decision rule that generates a large probability of error and does not improve when less pernicious conditions prevail.