Employing Raman gain of optical materials is appealing for a variety of laser applications, to include fiber laser combiners. In order for a Raman combiner to be efficient, the Raman material must have high gain, low loss figure at 1st Stokes wavelength, and high loss figures at higher order Stokes wavelengths. This paper demonstrates an efficient double-clad fiber Raman combiner utilizing fused silica core as gain material with microstructured cladding designed with filtering properties implemented for suppression of higher order Stokes propagation in the core. Comprehensive study results of this Raman combiner will be presented.
Robust monitoring at a carbon capture and storage (CCS) site to detect, locate, and quantify CO2 migration is necessary for providing early warning for an approaching well failure and potential gas leaking. We report on distribute chemical sensing via Raman spectroscopy in hollow core fibers (HoF) for direct gas detection via the percolation of gas into the fiber open core, leading to larger matter-light interaction and thus amplification of Raman signal. We present our experimental results on Raman detection in terms of concentration and uptake time for various HoF lengths with and without side microchannels. Guided by numerical studies, the optimized number of side holes lasers to possibly augment CO2 penetration rate into the air-filled core was determined and channels were drilled with pulsed femtoseconds Ti-Sa laser. We also investigated splicing open joint collars for integrating HoFs with solid core fibers (SCFs), critical in any deep or large surface area coverage deployment, and fiber Bragg gratings (FBGs), valuable in enhancing SNR via backscattering while generating time-space signal mapping and temperature/pressure sensing for baselining. By interleaving HOFs, FBG and SCFs, the optical spectroscopy methodology could offer a path to overcome current roadblocks to gas storage wells, as specialized fiber optics will allow direct detection of gases, in wells with in-situ and low power measurements of concentration.
We successfully fabricated a dispersion-shifted 20-um-core LMA fiber by incorporating four resonant side cores. This fiber was designed to operate at 1640-nm eye-safe wavelength and has been used in a pulsed Raman amplifier to overcome modulational instability. A peak power of 100-kW was obtained from the Raman amplifier.
Shrinking the volumetric footprint of gas sensors is desirable as it allows for nonintrusive, nonperturbing gas mixture analysis and access to tight enclosures. Micro-resonators are a perfect candidate for these sensors as their size parameter (~micron) is minimal, and the typical surface propagating whispering gallery modes can interact with an analyte without disrupting the environment. The large, quality factor (Q) of these resonant cavity modes enables long interaction lengths on the order of 100s of centimeters between the optical field and analyte. Thus, the presence of a gas different than the nominal environment will result in a shift of the resonant properties, including the resonant wavelength, amplitude, and quality factor, that can be detected in real-time. To illustrate this effect, we utilized a spherical micro resonator on the end of a piece of optical fiber, formed using standard ball lens fabrication, and excited the resonant modes using a tapered optical fiber connected to tunable Infrared laser. The resonator was fixed in contact with the tapered region of fiber, and the assembly was placed inside an in-house, optically coupled, vacuum-tight vessel for gas testing. We compared the spectral response of air, pure CO2, and pure N2 gas, observing spectral shifting and broadening of the cavity resonances. In addition, the effect of vessel temperature on resonance peak position due to the thermo-optic effect was investigated and quantified. Lastly, a feedback arm was added to the setup to reduce signal noise and automated data analysis was implemented to improve data clarity.
This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of spherical micro-resonators, coupled to a symmetrically tapered optical fiber, as a gas sensor. We will discuss silica-based microspheres and optimized tapered fiber coupling systems to detect greenhouse gases, i.e. CO2 in this context. The coupling setup is designed to be portable and amenable to different controlled environments, from constrained and controlled geometries to open and flexible enclosures. 3D-printed spherical resonator and tapered-fiber holders were made to satisfy different requirements. We produced microspheres for absorption spectroscopy of targeted gas and fabricated tapers by HF etching, using an HF-resistant fixture for safer handling and reduced waste. Detection within loose enclosures was performed as a preliminary study, where we observed spectral shift and broadening in the cavity resonances induced by the gaseous environments. Optically coupled vacuum-tight vessels have been designed and built to understand environmental effects.
We present 10W single-mode fiber laser based on Nd+3 fiber operating at 1428nm. All-solid fused silica microstructured waveguide fiber design is employed to suppress amplification at 1μm. The Nd+3 fiber is pumped by commercial multi-mode 880nm diode.
We report the fabrication of extremely low NA preforms (<0.03), highly doped with Yb using a conventional Modified Chemical Vapor Deposition (MCVD) system. Our lowest NA preform (0.025 NA) was drawn to a 52um core step-index double-clad fiber operating in a single mode regime (M2=1.04). The fiber had a mode field diameter (MFD) and an effective area (Aeff) greater than 35um and 1000um2 respectively. In a fiber laser configuration, the efficiency was greater than 85% without any sign of photodarkening. To the best of our knowledge, by using our extremely low NA preforms we have demonstrated the largest MFD and Aeff to date for a single-mode step index double–clad Yb doped fiber without involving any micro-structuration.
We present a 3kW single-mode fiber laser based on an Yb-doped LMA fiber operating at 1080nm. The laser which is pumped by 9xxnm diode bars stacks. It is believed to be the highest power direct diode pumped single-mode fiber laser oscillator to date.
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.
Progress is being made developing monolithic, all-fiber 2μm wavelength devices that operate robustly at higher power
levels. This development includes the critical Tm-doped LMA fiber technology, compatible components such as pump
combiners and couplers, along with the optimization of high brightness, high efficiency 790nm pump diodes. In this
paper we present recent CW power scaling results and demonstrate a monolithic MOPA system operating at 400W
output power with around 20% E-O efficiency.
We report our recent progress in the design and fabrication of a completely monolithic linearly-polarized pulsed Yb-doped fiber laser, with >10kW peak power, tunable 2ns-0.2μs pulse duration, tunable 50kHz-50MHz repetition rate and 50W average power in a diffraction-limited, linearly polarized and stabilized 0.8nm line-width output beam operating at 1064nm. The innovative all-fiber design of the laser is desirable for deployment in industrial applications. A wide range of independently-tunable pulse durations and repetition rates make this laser capable to address a large variety of laser applications, including high-power nonlinear wavelength conversion processes, LIDAR, etc.
We report on the recent progress in the design and development of completely monolithic linearly-polarized pulsed
fiber amplifiers seeded by Q-switched fiber laser oscillators. We demonstrate near diffraction limited beam quality with
~ 20 kW peak power (1mJ pulse energy, ~ 45 nsec) pulses and an average power ~ 20 W at 20 kHz repetition rate with
linearly polarized (> 17dB PER) output from a simple MOPA design. The laser produces spectrally narrow pulses with
~ 0.5 nm linewidth centered at 1064nm, suitable for various non-linear applications including generation of visible and
UV light. The simple MOPA design consists of a monolithic fiber amplifier based on an optimized coil of polarization
maintaining large mode area (PM-LMA) fiber with 30 &mgr;m core and low power Q-switched fiber oscillator. Excellent
output beam quality is achieved through the mode selectivity of the coiled PM-LMA fiber in the amplifier stage. Such
compact and robust fiber lasers are suitable for a variety of applications, such as nonlinear wavelength conversion
processes using a variety of nonlinear materials, laser radars, etc.
A linearly-polarized, 977 nm pulsed laser capable of 1 kW, 15 μJ output has been demonstrated. The laser is based on Yb3+-doped fiber technology, is core pumped and has a monolithic, all-fiber design. A 13 dB polarization extinction ratio was observed at the maximum measured output power. The output performance of the laser is pump-limited and shows no sign of non-linear effects at the demonstrated output powers. The laser emission is inherently near-diffraction limited due to the single-mode nature of the fibers used.
We report our recent progress in designing and manufacturing new, completely monolithic, linearly polarized, continuous wave (CW) fiber lasers that provide more than 300W of output power in a near diffraction limited, single transverse mode, spectrally stabilized output beam having a narrow line-width. The demonstrated design is simple and practical: the monolithic laser cavity may consist of only a coil of polarization maintaining (PM), large mode area (LMA) active fiber having a fiber Bragg grating (FBG) at one end and a fiber cleave at the other end. Proper selection of the coil diameter enables gain in only one polarization mode so as to provide the linearly polarized output. Fiber lasers built using this novel technique do not require any external polarizing components or the use of polarizing fiber. Such compact and robust fiber lasers are suitable for a variety of applications, such as multi-kW power scaling through coherent beam combining, nonlinear wavelength conversion processes using a variety of nonlinear materials, etc.
In this paper we review the damage mechanisms that need to be considered when building high power fibre lasers. More specifically we look at thermal issues, optically induced coating damage, bulk and surface damage thresholds of the host glass. We also discuss the reliability of tapered fibre bundles and Bragg gratings at these power densities.
Fiber lasers have a number of distinct advantages over their more conventional solid state laser alternatives. These advantages include size, reliability, wavelength selectivity, heat dissipation, wall plug efficiency and operational cost. Furthermore they can be operated without the need for active cooling or optical alignment. Consequently the market for these more traditional laser sources are beginning to be eroded by the emergence of fiber lasers. In 1999 high power fiber lasers became a reality, with the world's first single-mode fiber laser exhibiting in excess of 100W cw output. However it was soon recognized that conventional small core, high NA fiber designs were not appropriate to applications requiring further scaling of the output power. More specifically it was found that the maximum achievable output power in such fibers were restricted by a fundamental susceptibility to optical nonlinearities, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and self-phase modulation. In order to overcome the limitations imposed by these parasitic nonlinear processes, it has been necessary to develop fibers with high rare-earth dopant concentrations in relatively large core, low numerical aperture fibers. These so-called large mode area (LMA) fibers are directly responsible for the recent explosion in demonstrated diffraction-limited beam quality output powers, now approaching the kW-level from a single fiber. To further scale the output power it is necessary to combine the output of several fiber lasers. Indeed, for a number of industrial and military applications it is desirable to scale the total output power to between several and hundreds of kW's. It is therefore advantageous to be able to coherently combine the beams from multiple fibers and this it turns makes it desirable for the fiber to also be polarization maintaining. This provides yet another layer of complexity to the fiber design but such fibers are now a commercial reality. In this paper we review the recent and ongoing advantages in fiber design that is facilitating the development and production of lasers and amplifiers with ever increasing output powers.
The advent of double clad fiber technology has made high power lasers and amplifiers possible. However, the scalability of output powers can be limited by amplified spontaneous emission and nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using low numerical apertures (NAs), large-mode areas (LMAs), novel index profiles and high dopant concentrations. This paper discusses advances made in design and fabrication of highly efficient, large-mode area double clad fibers. Experimental and modeling results pertaining to changes in mode area, resultant power density and nonlinear threshold with changing core size are discussed. In addition, the mechanical reliability of the LMA fibers is discussed.
Fiber reinforced epoxy resins manufactured in autoclaves are expected to continue to dominate the composites market through 2010. However, the ability to obtain consistent mechanical properties from product-to-product remains difficult. This is largely due to the inability to monitor and control epoxy cure, loosely defined as the process of chain extension and cross-linking. Current autoclave process control employs a heat schedule based on a time-temperature-transformation (TTT) phase diagram that is determined by dynamic mechanical rheology. The phase diagram defines epoxy cure in terms of gelation and vitrification. We have been using an FT-Raman spectrometer to develop correlations between molecular (chain extension and cross-linking) and macroscopic (gelation and vitrification) data. The basis of a TTT phase diagram using Raman kinetic data for process control will be presented for several reactions.
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