This PDF file contains the front matter associated with SPIE Proceedings Volume 7686, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Tetragonal NaT(WO₄)₂, T= trivalent Y, La, Gd and Lu, single crystals doped with Yb³⁺ or Tm³⁺ have shown efficient room temperature laser operation at λ≈1.05 μm and λ≈1.95 μm, respectively. The broad bandwidth of the optical transitions of these lanthanides is of particular interest for diode-laser-pumped tunable and mode-locked femtosecond lasers. The present knowledge about these crystals and their applications as solid state lasers is overviewed. Results of new material preparation directions to produce epilayers and nano-, micro-particles of these compounds are described.

We present a Nd:YVO₄ Q-switched oscillator with spectrally controlled output operating at repetition rates up to 25 kHz and providing energy up to 1.5 mJ in 50 ns pulses. Spectral control is provided by an intra-cavity volume Bragg grating (VBG), which locks the output wavelength to 1064.24 nm and narrows the spectral linewidth to less than 10 pm. The side-pumped geometry of the gain medium allows for controlled spatial profile and compactness (30x60 cm²) of the total setup.

A cryogenically cooled Yb:YLF laser with 224-W output power at 995 nm, linearly polarized along the c-axis, has been demonstrated, and laser oscillation has also been obtained polarized along the a-axis. The beam quality had an M² ~ 1.1 at 60-W output and M2 ~ 2.6 at 180-W output for c-axis polarization. This level of average power is approximately two orders of magnitude higher than demonstrated previously in cryogenic Yb:YLF. A cryogenic Yb:YLF mode-locked oscillator is under development, which will be used to as the input to a Yb:YLF amplifier to generate a short pulses at high average power.

Silicate glass based fiber lasers emitting in the shortwave infrared spectrum, i.e., 1.4 μm to 3 μm are based on the phonon-terminated transitions of Er, Tm and Ho. In this review I will examine the pump, fiber and dopant configurations that have been tested in order to maximize the efficiency and output power produced from these transitions and I will highlight some of the issues that may need to be addressed in order to further scale the output power.

We report an optical parametric oscillator (OPO) system operating at 1574 nm using KTP crystals, with output peak power of more than 5 megawatts, output pulse energy of up to 30 mJ per pulse, and pulse width of less than 6 nanoseconds at full width half maximum (FWHM). The OPO was pumped by a diode pumped Nd:YAG Q-switched laser, with pump energy of about 95 mJ and pulse width of approximately 7 ns. The conversion efficiency from 1064 nm Nd:YAG laser to OPO output at 1574 nm is more than 30%. The complete Nd:YAG / OPO system, compactly packed inside a case with foot print measuring 15" × 9" × 5.3", was tested over an operating temperature range of -20 °C to +35 °C and a storage temperature range of -40 °C to +50 °C without significant power or performance variations.

Beam combining of fiber lasers has attracted much interest as a practical means to power scale fiber laser/amplifiers beyond the limitations of a single mode output from an individual fiber [1]. Almost all of the high power demonstrations to date that deliver good beam quality after the combing process (coherent and spectral) require some linewidth control for efficient combining, typically less than 10GHz [2,3,4]. Previously we demonstrated single mode, Yb-doped LMA fiber amplifiers operated with around 7GHz linewidth at 1kW output power [5]. In this paper, the latest generations of these amplifiers, based on the latest developments in LMA Yb-doped fiber technology demonstrate the capability to operate with linewidths around 3GHz at the 1kW power level. We present the latest data on optical properties of these new Yb-doped amplifiers and the SBS threshold as a function of input seed laser linewidth and discuss the technologies being developed to operate at higher power levels and narrower linewidths.

In this work, resonant diode pumping has been demonstrated for Q-switched and CW Er:YAG solid state lasers (SSLs) at eye-safe wavelengths. Resonant pumping was realized by using high spectral brightness 1470 nm laser diodes. An efficient 1645 nm CW laser with output power >5 W in the TEM00 mode was demonstrated. Total optical-to-optical efficiency was >19%. More than 11 mJ of output pulse energy and an output peak power of ~400 kW have been achieved in the TEM00 mode for Q-switched operation.

We report on the design and characterization of a cryogenically cooled, resonantly pumped Er:YAG slab laser operating at 1645 nm. The Er:YAG slab is conductively cooled by liquid nitrogen and face-pumped by a 1.4 kW diode array operating at 1452 nm. The slab is transversely extracted in a highly multi-mode oscillator, producing 386 W of cw output power and 420 W of quasi-cw power at 50% duty cycle. We have measured 45% slope efficiency and 39% optical conversion efficiency (relative to incident pump). The laser has also been configured to operate with low-order multi-mode output, producing over 250 W of quasi-cw power at 25% duty cycle, and has been Q-switched at repetition rates as low as 10 kHz.

Lasers for remote sensing based on Nd:YAG and OPO or Raman technology for wavelength conversion are developed at Carl Zeiss Optronics for a variety of military applications. For the laser altimeter instrument on the BepiColombo mission of ESA a laser diode pumped Nd:YAG laser will be used to enable accurate mapping of the Mercury surface from a 400 km to 1500 km orbit. The laser design addresses the requirements for low power consumption, high reliability and long operational life for this space mission. This is combined with a high beam quality, a pulse duration of less than 8 ns and a wide operating temperature range. We present details on the technological solutions applied to this laser and preliminary results on measured laser parameters. A life time test with temperature cycling performed on the laser diode stacks is presented. The technology of this laser is applied to a military dual wavelength laser transmitter for target designation and ranging. This laser uses Nd:YAG for the designation wavelength and OPO wavelength conversion for the eyesafe ranging wavelength. Technical data on this laser as well as performance measurements are presented.

Traditional silica fibers currently are unlikely to be able to sustain the powers needed for future Air Force applications. The low thermal conductivity of silica makes it difficult to control thermal gradients within the fibers resulting in failure or degradation in beam quality. While some of these problems can be ameliorated by using longer fibers, this results in problems with nonlinear effects such as stimulated Raman and Brillouin scattering (SRS and SBS). Yttrium aluminum garnet (Y3Al5O12, YAG) has the potential for overcoming these problems due to 1) higher thermal conductivity, 2) reduced thermal lensing, and 3) higher SBS threshold. Polycrystalline YAG has been demonstrated to be a highly efficient and economical laser host material in slab form. Polycrystalline YAG can be doped more uniformly and at higher levels than single-crystals with no dopant loss by zone refinement, has higher fracture toughness than single-crystals, and supports higher power densities. Despite the anticipated advantages, polycrystalline YAG has never been demonstrated in high-power fiber lasers. The development and characterization of YAG fibers for high energy laser applications is the primary goal of our research. Recent results in the processing of optical quality polycrystalline YAG fibers will be presented and discussed.

We present spectroscopic properties and lasing results of Ho³⁺-doped Yttria (Y₂O₃), LuAG (Lu₃Al₅O₁₂), and YAG (Y₃Al₅O₁₂) at wavelengths beyond 1.6 μm. High resolution measurements of absorption and stimulated emission cross sections of Ho³⁺ in these hosts from 77K to 300K are reported. Laser operation based on ⁵I₇ to ⁵I₈ transitions of Ho³⁺ in these hosts is demonstrated.

The objective of this effort is to develop more reliable, higher efficiency diode pumped Nd:YAG laser systems for space applications by leveraging technology investments from the DoD and other commercial industries. Our goal is to design, build, test and demonstrate the effectiveness of combining 885 nm laser pump diodes and the use of ceramic Nd:YAG for future flight missions. The significant reduction in thermal loading on the gain medium by the use of 885 nm pump lasers will improve system efficiency.

Reported here is cryogenically-cooled performance of Er3+-doped Y2O3 laser at 2.7 μm based on ⁴I_11/2 ⇒ ⁴I_13/2 transitions and diode-pumped at 974 nm directly into an upper laser manifold ⁴I_11/2. Efficiency of cryogenically cooled performance is getting close to its quantum defect limited value of ~30%.

Over the past few decades, diode laser technology development has achieved remarkable improvement in power, reliability and efficiency. Spectral brightness, wavelength-stabilization and spatial brightness are becoming very important for pumping of novel solid-state gain media and fiber lasers especially for efficient and power-hungry industrial and military applications. We will discuss the benefits of using 975 nm narrow-band curved grating Surfaceemitting Distributed Feedback lasers for pumping fiber lasers and thin disk lasers. SE-DFB lasers with less than 0.25 nm emission bandwidth, 0.07nm/°C thermal wavelength drift with over 50% power conversion efficiency has been achieved with a single emitter producing 73 W of CW power. Two-dimensional arrays of these lasers have been made for power scaling to achieve 1kW of power with less than 1nm spectral bandwidth. We will discuss the results and key advantages of using spectrally and spatially bright diodes for pumping fiber and thin disk lasers.

High brightness laser diode arrays are increasingly found in defense applications either as efficient optical pumps or as direct energy sources. In many instances, duty cycles of 10- 20 % are required, together with precise optical collimation. System requirements are not always compatible with the use of microchannel based cooling, notwithstanding their remarkable efficiency. Simpler but effective solutions, which will not involve high fluid pressure drops as well as deionized water, are needed. The designer is faced with a number of challenges: effective heat removal, minimization of the built- in and operational stresses as well as precise and accurate fast axis collimation. In this article, we report on a novel laser diode array which includes an integral tap water cooling system. Robustness is achieved by all around hard solder bonding of passivated 940nm laser bars. Far field mapping of the beam, after accurate fast axis collimation will be presented. It will be shown that the design of water cooling channels , proper selection of package materials, careful design of fatigue sensitive parts and active collimation technique allow for long life time and reliability, while not compromising the laser diode array efficiency, optical power density ,brightness and compactness. Main performance characteristics are 150W/bar peak optical power, 10% duty cycle and more than 50% wall plug efficiency with less than 1° fast axis divergence. Lifetime of 0.5 Gshots with less than 10% power degradation has been proved. Additionally, the devices have successfully survived harsh environmental conditions such as thermal cycling of the coolant temperature and mechanical shocks.

We present state-of-the-art performance from green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we demonstrate high power, high efficiency, continuous-wave operation from single-lateral-mode electrically pumped laser diodes at wavelengths from 405 nm to 500 nm. Additionally, we present the longest reported continuous-wave lasing demonstration of 525 nm and an output power of over 9 mW at 521 nm. Wall-plug efficiencies of over 25% in the violet region, 17.5% in the blue region, over 5% at 472nm, and 2.2% in the 500 nm range are reported. These InGaN-based devices offer dramatic improvement in size, weight, and cost over conventional gas and solid state lasers and may enable a variety of new applications in defense and security.

Northrop Grumman Cutting Edge Optronics has developed a new laser diode array package with minimal bar-to-bar spacing. These High Density Stack (HDS) packages allow for a power density increase on the order of ~ 2.5x when compared to industry-standard arrays. This work contains an overview of the manufacturing process, as well as representative data for 5-, 10-, and 20-bar arrays. Near-field and power vs. current data is presented in each case. Power densities approaching 15 kW/cm² are presented. In addition, power and wavelength are presented as a function of pulse width in order to determine the acceptable operational parameters for this type of array. In the low repetition rate Nd:YAG pumping regime, all devices are shown to operate with relatively low junction temperatures. A discussion of future work is also presented, with a focus on extending the HDS architecture to reliable operation at 300W per bar. This will enable power densities of approximately 25 kW/cm².

High-power diode lasers in the mid-infrared wavelength range between 1.8μm and 2.3μm have emerged new possibilities either for direct military applications or as efficient pump sources for laser sources in the 2-4μm wavelength range for infrared countermeasures. GaSb based diode lasers are naturally predestinated for this wavelength range and offer clear advantages in comparison to InP based diode lasers in terms of output power and wall-plug efficiency. We will present results on MBE grown (AlGaIn)(AsSb) quantum-well diode laser single emitters with emitter widths between 90μm and 200μm. In addition laser bars with 20% or 30% fill factor have been processed. More than 30% maximum wall-plug efficiency in cw operation for single emitters and laser bars has been reached. Even at 2200nm more than 15W have been demonstrated with a 30% fill factor bar. Due to an increasing interest in pulsed operation modes for these mid-infrared lasers, we have investigated single emitters and laser bars at 1940nm for different pulse times and duty cycles. More than 9W have been measured at 30A with 500ns pulse time and 1% duty cycle without COMD for a single emitter. Most applications mentioned before need fiber coupled output power, therefore fiber coupled modules based on single emitters or laser bars have been developed. Single-emitter based modules show 600mW out of a 200μm core fiber with NA=0.22 at different wavelengths between 1870nm and 1940nm. At 2200nm an output power of 450mW ex fiber impressively demonstrates the potential of GaSb based diode lasers well beyond wavelengths of 2μm. Combining several laser bars, 20W out of a 600μm core fiber have been established at 1870nm. Finally for a 7 bar stack at 1870nm we have demonstrated more than 85W at 50A in qcw mode.

The diode pumped alkali vapor lasers are significantly beneficent from the developing of a new generation of highpower laser diode sources. The latest achievements in the technology of photo-thermo-refractive volume Bragg gratings opened new opportunities for the design and fabrication of compact external cavity laser diodes and bars with reflecting volume Bragg gratings as output couplers. A new developed fiber coupled 250W source consists from 7 channels of independently stabilized commercially available LD bars with standard AR-coatings at output facets. Using a specially designed reflecting volume Bragg grating, we demonstrated spectral narrowing of a single LD bar spectrum by over two orders of magnitude down to 16-18 pm at 780 nm wavelength. The volume Bragg laser bar output power exceeded 88% of that for the free-running laser bar. The spectral position of each LD precise tuning is made by means of a special method of temperature stabilization of an output volume Bragg coupler. Overall spectral width for whole system was less than 20 pm (<10 GHz). Optical pumping of a rubidium gain medium requires fine-tuning of pumping laser emission to precisely overlap with narrow Rb absorption band. The emission spectrum of the volume Bragg LD bar pumping source was tuned over a 300 pm spectral range without deteriorating the line-width. The absorption of the pump light by Rb atoms was measured in a rubidium vapor mixed with low pressure C2H6 buffer gas. More than 91% power of the pump source was absorbed by the low-pressure Rb vapor.

Recent progress and state of GaSb based type-I lasers emitting in spectral range from 2 to 3.5 μm is reviewed. For lasers emitting near 2 μm an optimization of waveguide core width and asymmetry allowed reduction of far field divergence angle down to 40-50 degrees which is important for improving coupling efficiency to optical fiber. As emission wavelength increases laser characteristics degrade due to insufficient hole confinement, increased Auger recombination and deteriorated transport through the waveguide layer. While Auger recombination is thought to be an ultimate limiting factor to the performance of these narrow bandgap interband lasers we demonstrate that continuous improvements in laser characteristics are still possible by increasing hole confinement and optimizing transport properties of the waveguide layer. We achieved 190, 170 and 50 mW of maximum CW power at 3.1, 3.2 and 3.32 μm wavelengths respectively. These are the highest CW powers reported to date in this spectral range and constitute 2.5-fold improvement compared to previously reported devices.

A maximum output power of 18.6 W has been measured at a pump power of 23.7 W in a Tm:fiber laser pumped adhesive-free bonded (AFB®) YAG/Ho:YAG/YAG composite. The corresponding optical-to-optical efficiency and slope efficiency are 78.5% and 81.2%, respectively. When the laser is operated in Q-switched mode, the shortest laser pulse width of 11 ns has been measured in our experiment at repetition rate of 5 KHz. The laser has been used as pump source of a mid-infrared ZGP OPO. The wavelength tuning range from 3.06 to 6.6 μm has been achieved in a 15-mm long ZGP crystal. The maximum output power is 1.44 W at pump power of 10 W with repetition rate of 5 KHz.

The near and mid-IR spectral region is of significant interest due to the atmospheric windows present in this region. Applications of lasers operating in this spectral region range from their use in remote sensing LIDAR, IR counter measures (IRCM), spectroscopy and Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) sensing systems. We report the development of a compact, highly efficient, high power intra-cavity pumped all-solid-state optical parametric oscillator (OPO) producing nanosecond pulses at kHz repetition rates with an output tunable from 1.5 microns to 3.4 microns with pulse energies up to 10mJ. With our novel Intra-Cavity OPO design, pump to signal conversion efficiencies up to 65% (which is very close to its quantum efficiency) at room temperature are achievable.

This paper discusses methods for characterization of high power lasers. Specifically, these methods have been developed for the High Energy Laser Joint Technology Office and used for independent, government-sponsored testing in the 25 and 100 kW phases of the Joint High Power Solid State Laser program. Primarily this paper addresses measurement of power and beam quality.

The United States had been at the forefront of technology, including crystal growth, from the mid 1900's until several years ago. The growth of crystalline materials is generally capital-intensive and low profit, with the value-added fabrication and thin film coating steps comprising the majority of the cost of the final optic. With the continuous improvements realized by scientists in foreign countries, many U.S. companies with crystal growth facilities are opting to procure material from outside the U.S. to boost profits. Compounded with Federal procurement regulations, the end result is that it has become difficult, if not impossible, to procure some mission-critical materials from U.S. sources, putting numerous DoD programs in potential jeopardy. In addition, there is a limited amount of research currently underway on new materials state-side. If the current trends hold, DoD programs will be at the mercy of foreign companies to supply crystalline materials which are mission critical to the DoD.

Since Krupke et.al. proposed and demonstrated pumping alkali atoms using diode lasers in 2003, there has been lot of interest in the diode pumped alkali laser (DPAL) systems. Several researchers have been able to scale the DPAL system to powers in the tens of watts. We have conducted a systems-level, weight-scaling study of a notional medium power, CW DPAL system. Three different modes of operation are considered: (i) very high pressure operation (over 25 atmospheres of He) in which the absorption and emission lines of the alkali atoms are broadened sufficiently to allow for efficient pumping with off-the-shelf diodes that have line width of 2 to 3 nm, (ii) intermediate pressure regime (~ 5 atmospheres) that requires diodes that are line narrowed to ~0.4 nm, and (iii) low pressure operation (~ 1 atmosphere) that requires diodes that are line narrowed to < 0.1 nm for efficient pumping of pump radiation into the alkali vapor. In the latter two cases some amount of methane, ethane, or some other gas would be needed to mix the two upper states rapidly; while in the first case, helium is used to broaden the transition and to mix the upper states. We have considered closed-cycle transverse flowing systems with the transverse length limited by medium inhomogeneity caused by heat deposition into the gas. Weight models have been developed for each of the following sub-systems: Pump Diodes, Fluid Flow System, Thermal Management System, Optics and Diagnostics System, Instrumentation & Control System, and Electrical Power system. The results of our weight estimates for a notional 100 kW DPAL system are presented.

We report on the fracture toughness as it is related to the fracture strength when different orientations of YAG single crystals are bonded to each other to make a selection of a mechanically desirable waveguide configuration. We have found that the K^IC (211//211) conforming twin is greater than K_IC (110//110) conforming twin that is about the same as K_IC of (111//111) twist 180° and flip 180° twins. All K_IC converge to the same value that is comparable if not greater than the non-composite counterparts¹ when the composite samples are fabricated using the standard AFB^R (Adhesive- Free Bond) process. A second equally important consideration is the refractive index difference as function of dopant concentration. We have developed a technique to measure the difference of refractive index between Er:YAG and undoped YAG, and between Nd:YAG, and undoped YAG. We have found that the specific refractive index variation for YAG is 4.22 × 10^-4 per 1% Nd dopant concentration, and is 2.08 × 10^-4 per 1% Er dopant concentration.

A developed formalism¹ for analyzing the power scaling of diffraction limited fiber lasers and amplifiers is applied to a wider range of materials. Limits considered include thermal rupture, thermal lensing, melting of the core, stimulated Raman scattering, stimulated Brillouin scattering, optical damage, bend induced limits on core diameter and limits to coupling of pump diode light into the fiber. For conventional fiber lasers based upon silica, the single aperture, diffraction limited power limit was found to be 36.6kW. This is a hard upper limit that results from an interaction of the stimulated Raman scattering with thermal lensing. This result is dependent only upon physical constants of the material and is independent of the core diameter or fiber length. Other materials will have different results both in terms of ultimate power out and which of the many limits is the determining factor in the results. Materials considered include silica doped with Tm and Er, YAG and YAG based ceramics and Yb doped phosphate glass. Pros and cons of the various materials and their current state of development will be assessed. In particular the impact of excess background loss on laser efficiency is discussed.

The power record for near-diffraction limited output from a single fiber laser amplifier is now 10 KW. However, a single fiber appears unlikely to approach powers greater than 100 kW, which is needed for some applications. Therefore, there is great interest in methods for combination of many high power fiber beams that maintain aggregate beam quality. A number of methods have been proposed, including active and passive phasing, and spectral combination. These methods have varying implementation and performance advantages. The limitations of these methods, and approaches to address them, are discussed as applied toward combination of kilowatt-class fiber amplifiers.

The quantum defect caused by the difference between the pump and laser photon energies results in heat generation, which deteriorates the performance of lasers. This effect is very significant in high power lasers, since it can cause the stress-induced refractive index change and temperature-induced gain change. The radiation-balanced technique, in which all photons generated in the laser cycle are annihilated with the cooling cycle, has been proposed to solve the problem in the case of solid-state lasers. Unfortunately, in the radiation-balanced laser the radiated energy increases only linearly with the length of the laser medium. We propose a radically new approach to solve the problem of heat generation in lasers by using a co-doped fiber laser with two pump sources. In this laser Yb³⁺ ions are responsible for the lasing process, and Tm³⁺ ions serve as a cooler incorporated in the body of the laser. This new technique provides an exponential growth of radiation along the laser medium leading to almost athermal operation.

Five-channel spectral beam combining (SBC) using volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass with 0.5 nm spectral separation between channels and combined power >750 W has been recently reported. We report on improvements in this technique with the use of thermal control of VBGs that allows precise high-power alignment required for dense SBC with 0.25 nm spectral separation of channels. Experimental results of passive coherent beam combining (CBC) of fiber lasers using multiplexed VBGs are presented and analyzed. Methods for achieving 100 kW class systems using novel hybrid architectures that combine both coherent and spectral beam combining are discussed.

SCHOTT has developed eye-safe laser glasses for laser range finder and medical/ biophotonics applications. The development described herein covers various combinations of key ions, Er, Yb, and Cr, with and without Ce, at controlled ratios and their perspective reduction - oxidation (REDOX) states to improve glass lasing, thermal lensing, and thermo-mechanical stability for field-based applications under high repetition rate operation. This report covers glass property characterizations and selective modeling results using statistically designed compositions.

Fiber lasers create unique opportunities for creating high energy lasers. The distributed gain and heat deposition, and the flexible resonator provide the means for scaling to high powers. In addition and perhaps more valuable is the idea that fiber lasers allow the creation of an extensible architecture: an architecture where the individual components can be researched, designed, improved and replaced independently. In order to create sources at power levels over 3kW in volumes less than .01m3/kW, weighing less than 2kg/kW at costs under $1 per Watt of fiber laser output, serious consideration first needs to be given to the underlying architecture of choice. In this presentation, several architectural constraints along with competing approaches will be presented. Preliminary results from high brightness fiber coupling, and fiber combiner designs and experiments will be presented.

Compact and high current laser diode drivers for pumping solid-state lasers have been developed and tested. Designed to operate from a single DL123 battery or equivalent, the OptiSwitch PLDD-150-1-1 delivers 150 A of peak current for 300 μs to a laser diode bar at a 1 Hz repetition rate. Measuring only 2.1 × 0.75 × 0.78 inches and weighing 15.2 g, the unit is suited for man-portable target designation, rangefinding, illumination, and remote sensing applications. This paper will discuss the design philosophy behind this class of drivers which offer peak currents up to 200 A plus lifetime testing of eight drivers all operating at elevated input voltage and temperature at 4.5 Hz for 10M shots without a single failure or degradation in performance. Lastly, temperature testing down to -40 degC will be discussed.

In recent years, a key approach for SBS suppression in fiber amplifiers for high power single frequency fiber lasers is based on designs of transverse acoustic properties in fibers. Although this approach provides some SBS suppressions, our new analysis considering the often omitted leaky acoustic modes demonstrates that it is fundamentally limited to SBS threshold increase of less than ten-fold in the most ideal case. We have established an acoustic mode solver based on simultaneous longitudinal and shear acoustic wave equations and rigorous boundary conditions. This enables us not only the capability of studying designs of arbitrary acoustic velocity profiles, but also finding leaky acoustic modes with their waveguide losses. In this work, we will demonstrate the applicability of acoustic modes and impact of leaky acoustic modes in SBS in fibers, especially those with transverse acoustic velocity profiles designed to suppress SBS. With consideration of the leaky acoustic modes, we have found that design of transverse acoustic velocity profiles only has a very limited SBS suppression in fibers with even the most optimized acoustic anti-guides. Longitudinal acoustic property profiling, on the other hand, can potentially provide well over two-orders of magnitude SBS suppression.