We analyze the scalability of diffraction-limited fiber lasers considering thermal, non-linear, damage and pump coupling limits as well as fiber mode field diameter (MFD) restrictions. We derive new general relationships based upon practical considerations. Our analysis shows that if the fiber's MFD could be increased arbitrarily, 36 kW of power could be obtained with diffraction-limited quality from a fiber laser or amplifier. This power limit is determined by thermal and non-linear limits that combine to prevent further power scaling, irrespective of increases in mode size. However, limits to the scaling of the MFD may restrict fiber lasers to lower output powers.

Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power

The composition and properties of neodymium-doped (Nd-doped) phosphate glasses used for simultaneous high-energy (10 3 –10 6 J) and high-peak-power (10 12 –10 15 W) laser applications such as fusion energy research, are reviewed. The most common base glasses are meta-phosphates (O/P ∼3) with the approximate composition: 60P 2 O 5 –10Al 2 O 3 –30M 2 O/MO; K/Ba or K/Mg are typical modifiers. The spectroscopy of Nd 3+ in these glasses is well understood and laser properties can be accurately determined from measured spectroscopic properties. The major mechanisms for Nd 3+ non-radiative relaxation are reviewed and empirical expressions are presented that predict these effects in phosphate glasses. Optical and thermal–mechanical properties have been measured on a number of laser glasses and can be correlated with composition. Sub-critical crack growth rates in stress regions I, II and III have been reported for the first time in phosphate laser glasses. The mechanism for Pt inclusion formation and dissolution has been studied leading to damage resistant (Pt-inclusion-free) laser glasses.

Nd-doped phosphate glasses for high-energy/high-peak-power lasers

In optimizing passively Q-switched lasers, there is a unique choice of output coupler and unsaturated absorber transmission which maximizes the laser output energy and efficiency for each three-way combination of laser gain medium, absorber medium, and pump intensity (i.e, inversion density). In the present paper, we generalize and solve the three coupled differential equations which describe the passively Q-switched laser to obtain closed form solutions for key laser parameters such as the output energy and pulsewidth. We then apply the Lagrange multiplier technique to determine the optimum mirror reflectivities and unsaturated absorber transmissions as a function of two dimensionless variables. The first variable, z, corresponds to the ratio of the logarithmic round-trip small signal gain to the round-trip dissipative (nonuseful) optical loss and is identical to that which was used in previous treatments to optimize the rapidly Q-switched laser. The second variable, /spl alpha/, is unique to the passively Q-switched laser and is equal to the saturation energy density of the amplifying medium divided by the saturation energy density of the absorber. It is largely determined by the ration of the absorber to stimulated emission cross sections, but also depends on the speed of relaxation mechanisms in the amplifying and absorbing media relative to the resonator photon decay time. Several design curves, valid for all four level amplifying and absorbing media, are then generated. These permit the design of an optimum passively Q-switched laser and an estimate of its key performance parameters to be obtained quickly with the aid of a simple hand calculator. In the limit of large /spl alpha/ (>10), the design curves are virtually indistinguishable from the rapidly Q-switched case. The curves can also be used to perform rapid tradeoff studies of different absorbing materials. The theory can also be applied to CW-pumped, repetitively Q-switched systems through a simple multiplicative factor for the laser gain. The theory is applied to the analysis of a passively Q-switched Nd:YAG laser previously reported in the literature and shown to give excellent agreement with the experimental results. >

Optimization of passively Q-switched lasers

The composition and properties of neodymium-doped (Nd-doped) phosphate glasses used for simultaneous highenergy (10 3 ‐10 6 J) and high-peak-power (10 12 ‐10 15 W) laser applications such as fusion energy research, are reviewed. The most common base glasses are meta-phosphates (O/P3) with the approximate composition: 60P2O5‐10Al2O3‐ 30M2O/MO; K/Ba or K/Mg are typical modifiers. The spectroscopy of Nd 3a in these glasses is well understood and laser properties can be accurately determined from measured spectroscopic properties. The major mechanisms for Nd 3a non-radiative relaxation are reviewed and empirical expressions are presented that predict these eAects in phosphate glasses. Optical and thermal‐mechanical properties have been measured on a number of laser glasses and can be correlated with composition. Sub-critical crack growth rates in stress regions I, II and III have been reported for the first time in phosphate laser glasses. The mechanism for Pt inclusion formation and dissolution has been studied leading to damage resistant (Pt-inclusion-free) laser glasses. ” 2000 Elsevier Science B.V. All rights reserved.

Section 3. Applications Nd-doped phosphate glasses for high-energy/high-peak-power lasers

Shape memory polymers (SMPs) are smart materials that can remember a primary shape and can return to this primary shape from a deformed secondary shape when given an appropriate stimulus. This property allows them to be delivered in a compact form via minimally invasive surgeries in humans, and deployed to achieve complex final shapes. Here we review the various biomedical applications of SMPs and the challenges they face with respect to actuation and biocompatibility. While shape memory behavior has been demonstrated with heat, light and chemical environment, here we focus our discussion on thermally stimulated SMPs.

Biomedical applications of thermally activated shape memory polymers

Spectroscopic, optical, and thermomechanical properties of gadolinium scandium gallium garnet doped with trivalent neodymium and/or chromium are reported for use in the design of high-power solid-state lasers

Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet

Aliphatic urethane polymers have been synthesized and characterized, using monomers with high molecular symmetry, in order to form amorphous networks with very uniform supermolecular structures which can be used as photo-thermally actuable shape memory polymers (SMPs) The monomers used include hexamethylene diisocyanate (HDI), trimethylhexamethylenediamine (TMHDI), N,N,N{prime},N{prime}-tetrakis(hydroxypropyl)ethylenediamine (HPED), triethanolamine (TEA), and 1,3-butanediol (BD) The new polymers were characterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, and tensile testing The resulting polymers were found to be single phase amorphous networks with very high gel fraction, excellent optical clarity, and extremely sharp single glass transitions in the range of 34 to 153 C Thermomechanical testing of these materials confirms their excellent shape memory behavior, high recovery force, and low mechanical hysteresis (especially on multiple cycles), effectively behaving as ideal elastomers above T{sub g} We believe these materials represent a new and potentially important class of SMPs, and should be especially useful in applications such as biomedical microdevices

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Shape memory polymers based on uniform aliphatic urethane networks

Measurements of the intrinsic thermomechanical and thermo-optical properties of the new laser material LiCaAlF6:Cr3+ (known as Cr:LiCAF) are performed. Thermal diffusivity, heat capacity, thermal expansion, elastic constants, fracture toughness, and dispersion and temperature variation of the refractive index are all characterized for this material. In addition, the magnitude of the thermal lensing induced in a flash-lamp-pumped laser rod of Cr:LiCAF is measured and compared with the results obtained for an alexandrite laser rod in the same laser head. We find that the thermal lensing of Cr:LiCAF is favorably small and that the thermomechanical properties are expected to be adequate for applications at low and medium average power.

Thermomechanical and thermo-optical properties of the LiCaAlF 6 :Cr 3+ laser material

A system for removing matter from a conduit. The system includes the steps of passing a transport vehicle and a shape memory polymer material through the conduit, transmitting energy to the shape memory polymer material for moving the shape memory polymer material from a first shape to a second and different shape, and withdrawing the transport vehicle and the shape memory polymer material through the conduit carrying the matter.

Shape memory polymer medical device

Compressive epitactic layers grown on single‐crystal substrates are shown to substantially improve mechanical durability. In this study, neodymium‐substituted gadolinium gallium garnet (GGG) layers are grown on undoped GGG substrates. The layers are found to dramatically improve the abrasion resistance of the substrates, but to have only a slight effect on strength. Abrasion treatments, which cause up to 20 times decrease in the strength of substrates without epitactic layers, do not cause a significant decrease in the strength of substrates with these compressive surface layers. This permits the high strength of specially prepared strong substrates to be retained after abrasion.

John E. Marion

Papers

Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet

Shape memory polymers based on uniform aliphatic urethane networks

Thermomechanical and thermo-optical properties of the LiCaAlF 6 :Cr 3+ laser material

Shape memory polymer medical device

Compressive epitactic layers on single‐crystal components for improved mechanical durability and strength