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Stimulated Brillouin scattering (SBS) is typically the lowest order nonlinearity encountered in ytterbium doped fiber amplifiers (YDFA), and the simplest means of suppressing it is though linewidth broadening from phase modulation. However, to maintain compatibility with beam combining techniques critical to scaling to high output powers, narrow linewidths are needed, and bandwidth efficient means of suppressing SBS are key to scaling to high powers. Here, the scalability of novel phase modulation techniques in combination with laser gain competition are explored. Ultimately, LGC is shown to improve the TMI threshold by 15%, and reduce the linewidth by a factor of 2.1. A 1.8 kW fiber amplifier with 7 Ghz linewidth is demonstrated.

A 1.1 kW CW fiber amplifier emitting at 1.95µm and phase modulated at 5 GHz, with 51% optical conversion efficiency and near diffraction limited beam quality (M2

The development of high-power fiber lasers is of great interest due to the advantages they offer relative to other laser technologies. Currently, the maximum power from a reportedly single-mode fiber amplifier stands at 10 kW. Though impressive, this power level was achieved at the cost of a large spectral linewidth, making the laser unsuitable for coherent or spectral beam combination techniques required to reach power levels necessary for airborne tactical applications. An effective approach in limiting the SBS effect is to insert an electro-optic phase modulator at the low-power end of a master oscillator power amplifier (MOPA) system. As a result, the optical power is spread among spectral sidebands; thus raising the overall SBS threshold of the amplifier. It is the purpose of this work to present a comprehensive numerical scheme that is based on the extended nonlinear Schrodinger equations that allows for accurate analysis of phase modulated fiber amplifier systems in relation to the group velocity dispersion and Kerr nonlinearities and their effect on the coherent beam combining efficiency. As such, we have simulated a high-power MOPA system modulated via filtered pseudo-random bit sequence format for different clock rates and power levels. We show that at clock rates of ≥30 GHz, the combination of GVD and self-phase modulation may lead to a drastic drop in beam combining efficiency at the multi-kW level. Furthermore, we extend our work to study the effect of cross-phase modulation where an amplifier is seeded with two laser sources.

In this paper we report the generation of flat top optical spectrum using an arbitrary waveform generator to increase the SBS threshold in high power optical fiber amplifiers. The optical spectrum consists of a number of discrete spectral lines, ranging from 16 to 380, within the bandwidth of 2GHz, corresponding to line spacing between 133 MHz and 5 MHz. These discrete spectral lines correspond to a PRBS pattern of n = 4 to n = 8. The SBS threshold and coherence properties of the flat top spectrum are measured and compared to that of the filtered PRBS in a kilowatt class fiber amplifier. It is experimentally demonstrated that for large frequency line spacing, the flat top spectrum significantly outperforms the corresponding filtered PRBS, but as the line spacing is decreased to less than the Brillouin bandwidth, the two modulation waveforms have similar enhancement factors in the SBS threshold due to the enhanced crosstalk between neighboring frequency components.

Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30μm) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 μm annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth ~ 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92×10-11 m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers

Polychromatic laser light can reduce speckle contrast in wavefront-sensing and imaging applications that use direct detection schemes. To help quantify the associated reduction in speckle contrast, this study investigates the accuracy and numerical efficiency of three separate wave-optics models that simulate the active illumination of extended objects with polychromatic laser light. The three separate models use spectral slicing, Monte Carlo averaging, and depth slicing, respectively, to simulate the laser-target interaction. The sampling requirements of all three models are discussed. Comparisons to analytical solutions and experimental data are made when possible. In general, the experiments and theory compare favorably with the models.

The design of Q-switched lasers capable of producing pulse widths of 100’s of picoseconds necessitates the cavity length be shorter than a few centimeters. Increasing the amount of energy extracted per pulse requires increasing the mode area of the resonator that for the same cavity length causes exciting higher order transverse modes and decreasing the brightness of the output radiation. To suppress the higher order modes of these multimode resonators while maintaining the compact cavity requires the use of intra-cavity angular filters. A novel Q-switched laser design is presented using transmitting Bragg gratings (TBGs) as angular filters to suppress the higher order transverse modes. The laser consists of a 5 mm thick slab of Nd:YAG, a 3 mm thick slab of Cr:YAG with a 20% transmission, one TBG aligned to suppress the higher order modes along the x-axis, and a 40% output coupler. The gratings are recorded in photo-thermo-refractive (PTR) glass, which has a high damage threshold that can withstand both the high peak powers and high average powers present within the resonator. Using a 4.1 mrad TBG in a 10.8 mm long resonator with an 800μm x 400 μm pump beam, a nearly diffraction limited beam quality of M2 = 1.3 is obtained in a 0.76 mJ pulse with a pulse width of 614 ps.

We report efficient coherent beam combining of five kilowatt-class fiber amplifiers with a diffractive optical element (DOE). Based on a master oscillator power amplifier (MOPA) configuration, the amplifiers were seeded with pseudo random phase modulated light. Each non-polarization maintaining fiber amplifier was optically path length matched and provides approximately 1.2 kW of near diffraction-limited output power (measured M2

We report results from two ~1.5 kW Yb-doped fiber amplifiers with comparable optical to optical efficiencies and linewidths. One amplifier utilized a fiber with a core diameter of 25 μm while the core diameter of the fiber utilized in the other amplifier was 20 μm. Stimulated Brillouin scattering (SBS) suppression in both cases was achieved through pseudo-random bit sequence (PRBS) phase modulation. While the power generated in the larger core fiber was modal instability (MI) limited, no sign of MI was observed in the smaller core fiber. This may allow us to utilize the higher MI threshold fiber to scale further while maintaining sufficiently narrow linewidth for beam combining. Furthermore, in a demonstration of the utility of applying a thermal gradient in conjunction with phase modulation to suppress SBS further, we report on a 1 kW amplifier driven at a PRBS clock rate of 2 GHz. Finally, we compare the coherent beam combining properties of amplifiers seeded with PRBS phase modulated sources to those seeded with white noise sources.

Laser gain competition was used in conjunction with external phase modulation techniques in order to investigate power scaling of narrow-linewidth monolithic Ytterbium-doped fiber amplifiers. In this study, both pseudo-random bit sequence (PRBS) and filtered white noise source (WNS) modulation techniques were separately utilized to drive the external phase modulator for linewidth broadening. The final-stage amplifier was then seeded with the phase modulated narrow-linewidth 1064 nm signal along with a spectrally broader 1038 nm source. Consequently, integration of laser gain competition in conjunction with PRBS phase modulation yields a factor of ∼15 dB in stimulated Brillouin scattering (SBS) threshold enhancement at a clock rate of 2.5 GHz; leading to 1 kilowatt of output power with 85% optical efficiency at 1064 nm. Notably, the combination of PRBS phase modulation with laser gain competition provided superior enhancement in SBS threshold power when compared to the WNS modulated case. The beam quality at maximum power was near the diffraction limit (M2

Power scaling using a higher order mode in a ribbon fiber has previously been proposed. However, methods of selecting the higher order mode and converting to a single lobe high brightness beam are needed. We propose using a multiplexed transmitting Bragg grating (MTBG) to convert a higher order mode into a single lobe beam. Using a ribbon fiber with core dimensions of 107.8 μm by 8.3 μm, we use the MTBG to select a higher order mode oscillating within the resonator with 51.4% efficiency, while simultaneously converting the higher order mode to a beam with diffraction limited divergence of 10.2 mrad containing 60% of the total power.

White noise phase modulation (WNS) and pseudo-random binary sequence phase modulation (PRBS) are effective techniques for mitigation of nonlinear effects such as stimulated Brillouin scattering (SBS); thereby paving the way for higher power narrow linewidth fiber amplifiers. However, detailed studies comparing both coherent beam combination and the SBS suppression of these phase modulation schemes have not been reported. In this study an active fiber cutback experiment is performed comparing the enhancement factor of a PRBS and WNS broadened seed as a function of linewidth and fiber length. Furthermore, two WNS and PRBS modulated fiber lasers are coherently combined to measure and compare the fringe visibility and coherence length as a function of optical path length difference. Notably, the discrete frequency comb of PRBS modulation provides a beam combining re-coherence effect where the lasers periodically come back into phase. Significantly, this may reduce path length matching complexity in coherently combined fiber laser systems.

Increasing the dimensions of a waveguide provides the simplest means of reducing detrimental nonlinear effects, but such systems are inherently multi-mode, reducing the brightness of the system. Furthermore, using rectangular dimensions allows for improved heat extraction, as well as uniform temperature profile within the core. We propose a method of using the angular acceptance of a transmitting Bragg grating (TBG) to filter the fundamental mode of a fiber laser resonator, and as a means to increase the brightness of multi-mode fiber laser. Numerical modeling is used to calculate the diffraction losses needed to suppress the higher order modes in a laser system with saturable gain. The model is tested by constructing an external cavity resonator using an ytterbium doped ribbon fiber with core dimensions of 107.8μm by 8.3μm as the active medium. We show that the TBG increases the beam quality of the system from M 2 = 11.3 to M 2 = 1.45, while reducing the slope efficiency from 76% to 53%, overall increasing the brightness by 5.1 times.

Power scaling of high power laser resonators is limited due to several nonlinear effects. Scaling to larger mode areas can offset these effects at the cost of decreased beam quality, limiting the brightness that can be achieved from the multi-mode system. In order to improve the brightness from such multi-mode systems, we present a method of transverse mode selection utilizing volume Bragg gratings (VBGs) as an angular filter, allowing for high beam quality from large mode area laser resonators. An overview of transverse mode selection using VBGs is given, with theoretical models showing the effect of the angular selectivity of transmitting VBGs on the resonator modes. Applications of this ideology to the design of laser resonators, with cavity designs and experimental results presented for three types of multimode solid state lasers: a Nd:YVO4 laser with 1 cm cavity length and 0.8 mm diameter beam with an M 2 of 1.1, a multimode diode with diffraction limited far field divergence in the slow axis, and a ribbon fiber laser with 13 cores showing M 2 improved from 11.3 to 1.5.

Volume Bragg gratings (VBG) recorded in photo-thermo refractive glass (PTR) have high stability, and high damage threshold, allowing for many applications to the design of high power lasers. Gratings recorded in the transmitting geometry (TBG) have narrow angular selectivity, and can be used as a spatial filter in a resonator. Such gratings have previously been useful for improving the brightness of high power diodes, and increasing the beam quality in rod geometry solid state lasers. As the gratings have narrow angular selectivity, losses for higher order modes in the resonator no longer depend on the cavity length, allowing for the construction of short cavities with large mode areas. In this paper, we explore the design of short 1cm cavities using two TBGs as a spatial filter and no aperture in the cavity. The M 2 parameter as a function of pump size and angular selectivity of the TBG are explored, using pump diameters ranging from 800um to 2mm and angular selectivity ranging from 11mrad to 1.8mrad. An M 2 parameter of 1.05 is reported for an 800μm pump diameter, a 6.2mrad TBG, and a 1cm long cavity.

Volume Bragg gratings have been successfully used in spectr al beam combining of high po wer fiber lasers with narrow channel separationand in four channel passive coherent beam combining of fiber lasers. Future application of beam combining with kilowatt level lasers requires a more detailed understanding of how to cool the gratings without hurting beam quality. Forced air cooling blown across both surfaces of the grating is both easy and cheap, but has been avoided in the past due to concerns of how the air density fluctuations will hurt beam quality. It is now shown that forced air cooling has no adverse effect on the M 2 parameter due to density fluctuations in the air, and can efficiently cool VBG’s such that no degradation in beam quality is seen due to thermal distortions. Volume Bragg gratings are routinely made with low absorption on the level of 10 -4 cm -1 . To model grating operation for kilowatt level lasers, absorption of the studied grating was artif icially increased by 140 times. Thus the use of 80 W fiber lasers enabled imitation of 11kW beams, and the effects of forced air cooling on the beam quality were measured. Without cooling, the M

The recent development of kW fiber laser sources makes the concept of laser systems operating at power levels from tens of kilowatts up to 100 -kilowatt levels a reality. The use of volume Bragg gratings for spectral beam combining is one approach to achieve that goal. To make such systems compact, lower the complexity an d minimize the induced thermal distortions we propose and demonstrate the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The multiplexed volume Bragg gratings (MVBGs) were recorded in photo -thermo refractive glass and three beams with total power of 420 W were successfully combined using one MVBG. The combining efficiency was 97% and there was no significant beam quality degradation. The results demonstrated that the approach of using m ultiplexed volume Bragg gratings for spectral beam combining is an excellent extension to the current state of the art combining techniques. Especially valuable is the capability to reduce the number of optical elements in the system and while be ing able t o manage the expected thermal load when kilowatt level sources are used for beam combining. Key Words : Spectral Beam Combining, Volume Bragg Gratings, Multiplexed Volume Bragg Gratings, High Power, Fiber Lasers

Data density has a crucial impact on the accuracy of Digital Elevation Models (DEMs). In this study, DEMs were created from a high point-density LIDAR dataset using the bare earth extraction module in Quick Terrain Modeler. Lower point-density LIDAR collects were simulated by randomly selecting points from the original dataset at a series of decreasing percentages. The DEMs created from the lower resolution datasets are compared to the original DEM. Results show a decrease in DEM accuracy as the resolution of the LIDAR dataset is reduced. Some analysis is made of the types of errors encountered in the lower resolution DEMs. It is also noted that the percentage of points classified as bare earth decreases as the resolution of the LIDAR dataset is reduced.

The goal of the Virtual Prototyping Environment (VPE) is to decrease product development time and costs and to increase quality and flexibility by providing continuous computer support for the development cycle. Virtual prototype are directly derived from integrated CAD- Systems and enriched with simulation data. In addition, the VPE supports cooperative teams by providing different Computer Supported Cooperative Work (CSCW) techniques of shared viewing environments in combination with further communication tools for long distance collaboration. One aspect of CSCW in VPE's is multi-user and multi-application Shared 3D Environments. In the shared environment model, 3D objects from different applications can be joined into one scene that can be viewed by different users with independent or shared camera positions, enabling the distribution of visualization tasks between smaller, flexible and more specialized applications. The underlying product model of the VPE is based on the STEP standard applied to a distributed object-oriented data management approach. Taking the requirements from CE and shared 3D environments into account, we enriched the application interface to the data management with high level concepts such as object versioning, migration and consistency management, and extended the product data model to include presentation and annotation aspects.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.