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We have studied experimentally and modelled theoretically the capture properties and transport mechanisms of electrons and holes in laser structures. We describe first the extreme case where the barrier thickness is very large: then semiclassical drift-diffusion equations may be applied and quantum-mechanical effects at the edge of the well are negligible. A second extreme case occurs when the barrier is narrow enough for quantum mechanics to apply fully. There, strong variations of the capture time with the well width are expected and observed. In real laser structures, dimensions are such that we are in an intermediate case and both aspects have to be taken into account. We give some typical values for diffusion/capture mechanism induced delay times in such a case.

The dynamics of photoexcited carriers in GaAs quantum wells and superlattices has been measured with femtosecond resolution using luminescence as well as pump-probe experiments. The electron capture mechanism shows well defined resonances with a time as short as 500 fs at room temperature for a well thickness of 60 AA. This gives strong evidence for the importance of the quantum mechanical, LO phonon-assisted capture mechanism predicted by theoretical calculations. In type-II superlattices, we show that the transfer of electrons from the GaAs layers to the AlAs layer is mediated by zone edge phonons (both LO and TA) in a way similar to intervalley scattering. The observed times range from 140 fs up to 30 ps depending on the thickness of the GaAs layer.

The electronic and vibrational Raman spectra of a single crystal of ${\mathrm{YbPO}}_{4}$ have been investigated. The ${\mathit{E}}_{\mathit{g}}$ phonon at 310 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is found to have an anomalously large Raman linewidth of 50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at room temperature. Particularly striking was the observation that this transition is split into several separate transitions that move apart in energy as the temperature is lowered. The magnitude of these effects is very large. These observations are interpreted on the basis of electron-phonon coupling, and phenomenological calculations are presented and compared with the experimental data.

Femtosecond optical pulses of 14-fs duration were used to investigate exciton dynamics in II-VI CdZnTe-ZnTe multiple quantum wells, at room temperature. An exciton ionization time of 110 fs was determined. The bleaching strength of 'cold' neutral excitons on excitons is found to be larger than that from the same density of 'cool' free electron-hole pairs. >

We report the simultaneous generation of two collinear synchronous mode-locked pulse trains, with central wavelength separations of 60 to 80 nm, from a single-cavity mode-locked Ti:sapphire laser. Near-transform-limited pulses of duration 50 to 70 fs were generated at each wavelength, with jitter of less than 20 fs between the two-color pulses. These pulses were directly used to generate infrared pulses centered at 9 μm, by difference-frequency mixing in an AgGaS2 crystal.

We report the amplification of a colliding-pulse mode-locking femtosecond laser to energies of 2 microJ per pulse at a repetition rate of 3 kHz in a two-stage double-confocal resonator dye amplifier pumped by a Q-switched diode-pumped frequency-doubled Nd:YLF laser. The amplified pulses were used to generate a continuum in both an ethylene glycol jet and a single-mode fiber. The spectrally broadened pulses from the fiber were compressed to a duration of 13 fs.

We experimentally examine a class of multipass amplifiers based on an open double confocal resonator. A preamplifier provides a gain of 800. An intermediate-stage amplifier produces pulse energies of 2 microJ for only 1.4 W of pump power at a repetition rate of 10 kHz. A third structure, which includes a four-prism sequence, provides on each pass adjustable forms of the four basic pulse-shaping mechanisms of saturable gain, saturable absorption, group-velocity dispersion, and self-phase modulation, as well as spectral filtering. This latter structure also reversibly lengthens the pulse duration in the gain medium from 50 fsec to over 450 fsec.

Traveling-wave amplification of a lambda = 1.53 microm signal with +22-dB gain is achieved at 295 K in an Er(3+)-doped single-mode fiber using a lambda = 514.5 nm pump source. The optimum fiber length for maximum gain is determined experimentally. A limit in signal-to-noise ratio that is due to concurrent amplification of spontaneous emission is observed. By cooling the fiber to 77 K, the amplifier gain is increased to +29 dB as a result of depopulation of the lower laser level.

We demonstrate that a combination of prisms and diffraction gratings can provide not only quadratic but also cubic phase compensation of ultrashort optical pulses. We obtain compressed pulses as short as 6 fsec.

Resonance enhancement factors of 10 to 100 have been observed for the electronic Raman scattering transitions in crystals of pure ErPO4 and 1% Er3+ diluted in LuPO4. The resonances were excited by the 488.0-nm line of an argon-ion laser. This line is nearly coincident with the crystal-field levels of the 4F7/2 multiplet of Er3+. The differences in relative intensities between the transitions in ErPO4 and those in 1% Er3+ in LuPO4 arise from shifts in the energy levels in the two systems and suggest that quantum-mechanical interference effects play an important role.

Continued improvements in the performance of a multi-kilometer, low-erbium-concentration fiber amplifier made by the modified chemical vapor deposition (MCVD) seed fiber method are discussed. Transparency for a -20-dBm signal at 1.55 mu m and fiber length of 11.8 km was achieved with 20 mW of 1.48- mu m copropagating pump power. The wavelength dependence of the gain for this signal level is reported, and the distribution in small signal gain along a 22-km length using an optical domain reflectometer (OTDR) was measured. Simulations are presented to show the effect of erbium concentration, input signal level, and pump power required for transparency of a 50-km span. >

The dry friction and wear behavior of toughened zirconias against hardened steel was studied using the Falex ring and block technique. Three experimental ZrO2-Y2O3 ceramics and two commercial ZrO2-MgO ceramics were investigated. Each ceramic was tested at 500 and 2000 rpm at normal loads in the range 2.3 to 40.8 kg. Significant trends in the friction and wear data were found, correlating composition, test speeds, and loads. Microstructural examination of the ring, ceramic block, and wear debris has shown that the wear process is very complex and incorporates a number of mechanisms. 16 references.

We report the gain, noise figure, output saturation power, and conversion efficiency of a highly concentrated Er/sup 3+/-doped alumino silicate fiber amplifier. We obtain a gain per unit length of 1.0 dB/cm, which corresponds to the highest gain per unit length obtained in an Er/sup 3+/-doped fiber amplifier. The pump power threshold ranges from 2 to 5 mW, depending on the fiber length.

We report on the gain characteristics of a low threshold (8 mW) Er/sup 3+/-doped planar optical waveguide amplifier. Net fiber to fiber gain of 4.5 dB is achieved at a signal wavelength of 1536 nm with 80 mW of 980-nm pump power. This device represents significant progress toward a planar amplifier module pumped by a single laser diode.

A remotely pumped erbium-doped fiber amplifier system is described that provides a record budget improvement over a non-remotely pumped system of 11 dB for 145 mW of pump power. The improvement over previously reported experiments is a result of increased pump power as well as careful optimization of the fibers utilized. >

Erbium‐doped indium oxide films were prepared by rf magnetron sputtering. The Er‐doped oxide films are conducting (or semiconducting) with a resistivity in the range of 10−3–103 Ω cm, and are optically active, i.e., show a clear room‐temperature photoluminescence at 1.54 μm, corresponding to intratransitions in Er3+ ions. Compared with the undoped indium oxide films, the erbium doping was found to have the effect of increasing the resistivity (up to two orders of magnitude) of the films, mainly via a reduction of carrier concentration. Postdeposition annealing in air ambient significantly enhances both the Er3+ luminescence and Hall mobility (up to 60 cm2/V s), and reduces the carrier concentration. Postdeposition annealing in reducing ambient (N2/H2), however, decreases the resistivity dramatically, mainly via an increase of carrier concentration (up to 1020 cm−3), and also enhances the Er3+ luminescence.

We report the output saturation power, excess noise factor, and system performance of a highly concentrated Er/sup 3+/-doped planar optical waveguide amplifier. The performance data demonstrate the potential usefulness of planar optical waveguide amplifiers in system applications. >

We report on the concentration- and pump-dependent lifetimes of the spontaneous emission in Er/sup 3+/-doped fibers and Er/sup 3+/-doped waveguides. In addition, we measure the concentration dependence of the 550-nm fluorescence due to excited state absorption (ESA). >

Si/SiO2 Fabry–Perot microcavities with rare‐earth‐doped SiO2 active regions are realized for the first time. Cavity‐quality factors exceeding Q=300 are achieved with structures consisting of two Si/SiO2 distributed Bragg reflectors and an Er‐implanted (λ/2) SiO2 active region. The room‐temperature photoluminescence intensity of the on‐axis emission is 1–2 orders of magnitude higher for resonant cavity structures as compared to structures without a cavity.

The pump wavelength dependence of the gain of an erbium-doped fiber amplifier pumped in the wavelength region 950-1000 nm is discussed. It is found that efficient gain at 1.557 mu m, between 25 and 28 dB for 17 mW of launched pump power, can be obtained for any wavelength pump in the range 965-985 nm. The optimum length of erbium fiber needed is found to vary as a function of pump wavelength. The gain as a function of signal wavelength is also investigated at these pump wavelengths. >

We demonstrate the modification of spontaneous emission of Er atoms implanted into the SiO2 active region of Si/SiO2 planar microcavities. The modification of spontaneous lifetime by a cavity at radio frequencies was first proposed by Purcell1 in 1946. Other authors have demonstrated that confocal resonators indeed can drastically modify the spontaneous emission lifetime of an atom within a resonator at millimeter wavelengths2. This concept has been extended to short planar cavities at optical wavelengths3–7 in the past few years. The theory of emission from planar microcavities has also been advancing4,8–11. In this and previous works12,13, we have demonstrated the microcavity-modified spontaneous emission from a rare earth atom in a solid-state host. This may have potential device applications, since Er emits at the wavelength of the loss-minimum in glass fibers. Primarily, however, these structures are ideal for demonstrating spontaneous emission enhancement, spectral narrowing, and modification of emission lifetime. The short effective cavity lengths, the narrow natural Er spectrum, and the lack of self-absorption by the Er-atoms within the cavity maximize the microcavity effects. Spontaneous emission enhancement at the cavity resonance wavelength of two orders of magnitude is demonstrated. We also compare the lifetime of the atoms in various thickness cavities to a comprehensive model of emission from a planar microcavity. Finally, we theoretically examine the effects of reducing the device area on the spontaneous emission factor β and the observed spectrum.

Two collinear and synchronous modelocked pulse trains, with central wavelength separation adjustable between 55 and 90 nm, were obtained from a single cavity modelocked Ti:sapphire laser. Near transform limited pulses of duration 50 to 70 fs were generated at each wavelength, with very small jitter between the two color pulses.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Short infrared pulses have been generated by nonlinear mixing of the wavelengths from a two color femtosecond modelocked Ti:Sapphire laser. The pulses can be tuned from 7 to 12 micrometers and are used to perform a direct measurement of the speed of a GaAs/Al0.2Ga0.8As quantum well infrared photodetector. The electrical response of the detector has rise and decay times of less than 95 ps and the FWHM is 115 ps.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Er-doped glass thin films are drawing increasing attention for optical amplifiers suitable for integrated optic configuration. We have investigated deposition of highly Er-doped (mid 1019 atoms/cm3 silica films using Rf magnetron sputtering. Erbium was doped into the host material by cosputtering techniques. Deposited films (0.5 to 1.2-micrometers thick) were characterized by photoluminescence, secondary ion mass spectroscopy, and fluorescence decay measurements. We have also deposited Er-doped three component silicate glass (SiO2 + Al2O3 + MgO) films to investigate the dependence of Er3+ luminescence on the host material's composition. Optimum anneal temperature for the silicate glass films was found to be lower than that for the silica films while the Er3+ luminescence intensity was slightly higher for the silicate glass case.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report the first demonstration of a CW, diode-pumped, silica fiber amplifier at 1.3 μm. A gain of 35 dB and an output power of +24.7 dBm have been obtained at 1.3 μm in germanosilicate fibers using a resonantly-cascaded Raman laser initiated by a diode-pumped Nd3+ cladding-pumped fiber laser.

The power budget of repeaterless transmission systems is usually limited by the output power of optical post amplifiers and the sensitivity of optically pre-amplified receivers. The use of an Erbium-doped fiber amplifier (EDFA) remotely pumped from the receiver end can significantly improve the power budget of such systems, without incorporating any electrically active components along the cable [1]-[4]. Recent experiments have demonstrated budget improvements, over a non-remotely pumped system, of 7.4 and 8.5 dB [2, 3] for pump powers of 100 and 125 mW respectively. In this paper, we describe a remotely pumped EDFA system that provides a record budget improvement of 11 dB for 145 mW of pump power. The improvement over the previously reported experiments is a result of increased pump power as well as optimization of the Erbium-doped fiber and of the transmission fiber placed between the remotely pumped EDFA and the optical pre-amplifier.

We have demonstrated a 75 % coupling efficiency from a monolithically integrated master oscillator power amplifier (MOPA) diode laser source into a single-mode optical fiber. This was then used to pump a Pr3+-doped fiber amplifier. A net fiber gain of 19 dB was demonstrated and a 4 dB receiver enhancement was observed when the Pr3+-doped fiber was used as a preamplifier in a 2.5 Gbit NRZ transmission experiment.

The dynamics of carrier capture in GaAs quantum wells have been measured with femtosecond resolution using pump and probe experiments under resonant excitation conditions. The electron capture mechanism at room temperature shows a resonance with a time as short as 650 fs for a well thickness of 58 \AA{}. This gives strong evidence for the importance of the quantum-mechanical LO-phonon-assisted capture mechanism predicted by theoretical calculations.

Highly Er-doped (∼ 1020 atoms/cm3) silicon and silica films were deposited by RF magnetron sputtering. Erbium was doped into the host material by co-sputtering technique. Deposited films (0.5 - 1.2 µm thick) were characterized by photoluminescence (PL), secondary ion mass spectroscopy (SIMS), and fluorescence decay measurements. Er-doped silica glass films show a strong, room-temperature luminescence at 1.54 µm wavelength. In contrast, Er-doped silicon films show a weak luminescence at room temperature. However, a big enhancement in the Er3+ luminescence was observed after a proper annealing, for example, 900 °C for 30 to 120 min in air ambient, resulting in the luminescence intensities comparable with that of the Er-doped silica films. This enhancement is attributed to the oxygen incorporation into the Si host film during the annealing, thus forming an Er-doped oxide layer on top of the film. The result suggests that Er-O bonding plays important role in forming optically active erbium ions. Erdoped, three-component silicate glass (SiO2 + A12O3 + MgO) films were also sputter deposited to investigate the dependence of Er3+ luminescence on the host material's composition. Even stronger luminescence was observed from the Er-doped, three component silicate glass films compared with the Er-doped SiO2 films.

Pr3+, due to its four level laser transition 1G4→3H5 has been shown to be a potential candidate for amplification at 1.3 µm. Pr3+-doped fluorozirconate glass (ZBLAN) fibers have recently been demonstrated to yield large (≈ 30 dB) gain at 1.31 µm.1 The pump power needed to invert the Pr3+ ion and produce an efficient amplifier has been, to date, several hundred mW. This is due in large part to the short lifetime of the 1G4 level, reported to be 110 ps in ZBLAN, which is limited by non-radiative decay.1 Improvements can be made by finding a host for which the 1G4 lifetime is longer.

Pulses from femtosecond oscillators such as a CPM benefit from amplification, as this produces the high peak powers necessary for many non-linear optical experiments. We report here high energy, multi kHz, amplification of 75-fs pulses at 620 nm from a CPM, using as a pump source a compact solid-state laser, a Q-switched diode-pumped frequency doubled Nd:YLF laser.

Most work on erbium-doped fiber amplifiers (EDFA) over the past few years has focussed on amplifiers pumped at 980 nm and at 1480 nm.1-3 Recently, however, there has been renewed interest for file 800-nm pump band.4 In this paper we present a comparative study of an EDFA pumped by a Ti:sapphire laser tuned to both 800 and 980 nm in order to illustrate the effects of excited-state absorption.5 The erbium-doped fiber used in the experiment has a 4-μm-diameter SiO2-Al2O3-GeO2 core with a relative index difference of 0.035 and a cutoff wavelength of ~815 nm. The erbium concentration is 200 ppm. Gain and saturation measurements were per formed on different lengths of fiber, pumped at 980 nm and at some arbitrary wavelength in the 810-nm region, the optimum wavelength having been found to increase with length. Figure 1 is a plot of normalized gain for λs = 1533 nm as a function of pump wavelength. The 3-dB bandwidth is 14 nm. The peak gain for this 12-m piece is obtained for λ p = 811 nm and was 25 dB for λ s = 1533 nm and 20 dB for λ s = 1553 nm. The maximum absorption, however, was found to be at 797 nm.

With the growing importance of erbium-doped fiber amplifiers for telecommunications applications, considerable effort has been devoted to minimizing the pump power requirements for producing high gain. The best reported results are for λp = 0.98 μm wavelength pumping1,2 with gain coefficient up to 4.9 dB/mW, and for λp = 1.48 μm pumping,3,4 with a lower maximum gain coefficient of 2.6 dB/mW. We report here a low threshold erbium-doped fiber amplifier which, when pumped at 1.48 μm, has a gain coefficient is 5.9 dB/mW, and, when pumped at 0.972 μm, an almost equal gain coefficient of 6.1 dB/mW.

Optical amplifiers will be key components for increasing the versatility of optical fiber communications as in-line multichannel repeaters and preamplifiers.1 Since the early investigation of Nd doped fiber amplifiers,2,3 developments in vapor-phase techniques for fiber fabrication4 led to a renewed interest in a large variety of rare earth doped fiber devices.5 In particular, Er doped fibers, amplifiers, and lasers operating around 1.54 µm (Ref. 6) gained increasing attention as new devices for optical communications in the 1.5-Mm communication window.7,8 Our purpose is to outline the state of the art of Er doped fiber amplifiers (EDFAs) and review the current device issues.

There is currently a need for a stable and widely wavelength tunable femtosecond laser source for time resolved spectroscopic investigations. One approach is to amplify CPM pulses and generate a broad spectral continuum which can be either used directly or reamplified to reach wavelengths other than the fundamental CPM wavelength of 620 nm. Due to the high intensities available with the amplified CPM and continuum, there is potential for parametric mixing in a variety of nonlinear crystals so as to obtain femtosecond pulses in the 1-20 μm wavelength range. We report here preliminary results of the successful mixing of a femtosecond amplified CPM pulse and a femtosecond continuum so as to generate pulses in the mid infrared, at a repetition rate of 8.5 kHz. "Seeded" parametric amplification has been used previously by Jedju and Rothnerg to generate pulses in the 2-5 μm wavelength range using 350 fs pulses at a 10 Hz repetition rate.1

A direct measurement of the speed of a GaAs/Al/sub 0.2/Ga/sub 0.8/As quantum well infra-red photodetector is made using fast pulses centred at 9.7 mu m. The electrical response of the detector has rise and decay times of less than 95 ps and the FWHM is 115 ps.

Praseodymium-doped ZBLAN fibre amplifiers pumped by high power M-MOPA laser diodes demonstrate output powers up to l7.1 dBm and small signal gain levels up to 26 dB at 1.31 /spl mu/m wavelength. Optical receiver preamplification using an M-MOPA pumped PDFA results in a 4 dB sensitivity improvement for a 2.5 Gbit/s transmission experiment.

Ultrashort mid-infrared pulses at a repetition rate of 82 MHz have been generated by difference-frequency mixing at the output of a single-cavity, two-color femtosecond Ti:sapphire laser. The center wavelengths of the midinfrared pulses are tunable from 7.5 to 12.5 microm. Their pulse durations range from 450 to 650 fs, as a function of the center wavelength.

Using a 6 fs optical pulse we have impulsively excited the entire manifold of Franck-Condon-connected vibronic levels for the S 0 to S 1 transition in an organic dye in solution. Periodic perturbations to the absorption spectrum are observed as a consequence of the nuclear motion brought about by the coherent excitation of the manifold of vibrational levels.

The smaller-than-anticipated asymmetry of the electronic Raman scattering in TmPO4 is related to the effects of virtual excited states of the type 4fN-1g. These effects almost exactly cancel the contributions coming from states belonging to 4fN+1d+or-1. Revised calculations for ErPO4 are presented.

The optical and near infrared absorption of dilute Tm3+ and Yb3+ impurities in YPO4 and LuPO4 single crystals have been measured at liquid helium and nitrogen temperatures. For Tm3+, the spectral region from 5 000 to 38 000 cm−1 was examined and Zeeman spectra were obtained in the visible region. The observed transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least‐squares procedure. Satisfactory fits and good agreement between the calculated and measured g values were obtained. For Yb3+, there are more parameters than experimental levels, so ζ, B20, B40, and B44 were adjusted, while B60 and B64 were fixed at the values found for Tm3+. Energy levels and Zeeman splittings calculated with these parameters are in good agreement with the measured quantities.

During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.

Microstructural changes around non-metallic inclusions (sometimes called ‘butterflies') were studied using both optical and transmission electron microscopy. Large numbers of cyclic stresses cause the parent tempered martensite to disappear, along with the spherical cementite particles generally found in these through-hardened steels. The microstructural changes are described and the transformation products identified. Also, comparisons are made with similar events around crack tips, and a formation mechanism is proposed.

The relative intensities of the electronic Raman scattering between individual crystal-field states of the 4${f}^{1}$ configuration of ${\mathrm{Ce}}^{3+}$ in ${\mathrm{LuPO}}_{4}$ are compared to those calculated with use of the standard second-order theory, and also by explicitly evaluating the sum over the virtual intermediate states using the crystal-field wave functions and observed energies of the 5${d}^{1}$ configuration. The results show that the explicit calculation predicts the observed relative intensities much more accurately than the standard theory. In addition, a change in the incident laser energy from the argon-ion green line (514.5 nm) to that of the frequency-tripled output of a ${\mathrm{Nd}}^{3+}$:YAG (355 nm) laser results in enhancements of the electronic Raman scattering intensities by factors on the order of 100. These enhancements are accurately predicted by the explicit calculation.