John R. Peele

Noncontact fluorescence thermometry of acoustically levitated waterdrops

Noncontact thermometry based on the fluorescence excitation spectrum of aqueous Eu(3+) (EDTA) near 579 nm allows us to measure the temperature of an evaporating drop of water. The results for drop diameters in the 500-microm-3-mm range confirm theoretical temperature predictions for steady state evaporation. Calibration of the excitation spectrum in a constant temperature cell indicates that +/-1.0 degrees C resolution is possible for temperatures below 20 degrees C. The spectrum depends only on the solution temperature when Eu(3+) (EDTA) concentrations are below 1 x 10(-3) M and when the solution pH is between 4.0 and 10.0. Excitation spectra from levitated waterdrops contain additional noise which degrades the temperature resolution to +/-1.2 degrees C. With this technique we are able to follow the temperature change in an evaporating drop of water as a monolayer of 1-octadecanol forms on the surface and retards the evaporation.

Optical fiber sensors for spacecraft: applications and challenges

Optical fiber sensors offer a number of advantages for spacecraft applications, including freedom from electromagnetic interference, light weight, and low power consumption. One application is strain sensing, where high sensitivity and bandwidth and the ability to individually interrogate tens of multiplexed sensors via a single fiber lead has been demonstrated. This paper will describe 2 recent NRL uses of distributed strain sensing using arrays of fiber Bragg gratings (FBGs) on spacecraft parts, structures, and ground test hardware: distributed dynamic strain monitoring of a lightweight reflector during acoustic qualification tests and high-frequency, high-sensitivity strain measurements of a latch fixture. A second fiber sensor being seriously considered for spacecraft is the interferometric fiber optic gyroscope (IFOG). Although its performance in a benign environment is quite attractive, deployment of this and other optical fiber sensors requires addressing issues such as the deleterious effects of the space radiation environment. These challenges, unique to this application, will be discussed.

Inscription of Fiber Bragg Gratings in Multicore Fiber

We present solutions to unique problems associated with simultaneously writing FBGs in 4 cores of a multicore fiber used for bend and twist sensing, including exposure conditions, photosensitivity balance, and draw-induced refractive index changes.

Ho-doped fiber for high energy laser applications

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho3+ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.1

Characterization of strong fiber Bragg gratings using an applied thermal chirp and iterative algorithm

Coupling coefficients of various grating types and strengths are calculated from measurements of the complex reflectivity using an applied thermal chirp and optical frequency domain reflectometry (OFDR). The complex reflectivity is then utilized by a layer peeling algorithm to determine the coupling coefficient of the thermally chirped grating. A guess of the temperature profile enables the coupling coefficient of the unchirped grating to be estimated. An iterative algorithm is then used to converge on the exact coupling coefficient, employing an error minimization method applied to the reflectivity spectra. This technique removes the need for a reference grating while preserving the spatial resolution obtained with the initial OFDR measurement. Successful reconstruction of gratings with integrated |κ|L ~ 9.0 are demonstrated with a spatial resolution of less than 100 μm.

Nanoparticle doping for improved Er-doped fiber lasers

A nanoparticle (NP) doping technique was used for making erbium-doped fibers (EDFs) for high energy lasers. The nanoparticles were doped into the silica soot of preforms, which were drawn into fibers. The Er luminescence lifetimes of the NP-doped cores are longer than those of corresponding solution-doped silica, and substantially less Al is incorporated into the NP-doped cores. Optical-to-optical slope efficiencies of greater than 71% have been measured. Initial investigations of stimulated Brillouin scattering (SBS) have indicated that SBS suppression is achieved by NP doping, where we observed a low intrinsic Brillouin gain coefficient, of ~1× 10-11 m/W and the Brillouin bandwidth was increased by 2.5x compared to fused silica.

Erbium nanoparticle doped fibers for efficient, resonantly-pumped Er-doped fiber lasers

Nanoparticle (NP) doping is a new technique for making erbium-doped fibers (EDFs); the Er ions are surrounded by a cage of aluminum and oxygen ions, substantially reducing Er3+ ion-ion energy exchange and its deleterious effects on laser performance. Er-Al-doped NPs have been synthesized and doped in-situ into the silica soot of the preform core. We report the first known measurements of NP-doped EDFs in a resonantly-core pumped master oscillator-power amplifier (MOPA) configuration; the optical-to-optical slope efficiency was 80.4%, which we believe is a record for this type of fiber.

Cladding Pumped Single Crystal Yb:YAG Fiber Amplifier

We report on fabrication and optical properties of double clad single crystal Yb:YAG core fiber. For the first time, net gain is demonstrated in a cladding pumped Yb:YAG single crystal fiber structure.

Recent advancements in transparent ceramics and crystal fibers for high power lasers

In this paper, we present our recent progress in the development of rare-earth (Yb3+ or Ho3+) doped Lu2O3 and Y2O3 sesquioxides for high power solid state lasers. We have fabricated high quality transparent ceramics using nano-powders synthesized by a co-precipitation method. This was accomplished by developments in high purity powder synthesis and low temperature scalable sintering technology developed at NRL. The optical, spectral and morphological properties as well as the lasing performance from our highly transparent ceramics are presented. In the second part of the paper, we discuss our recent research effort in developing cladded-single crystal fibers for high power single frequency fiber lasers has the potential to significantly exceed the capabilities of existing silica fiber based lasers. Single crystal fiber cores with diameters as small as 35μm have been drawn using high purity rare earth doped ceramic or single crystal feed rods by the Laser Heated Pedestal Growth (LHPG) process. Our recent results on the development of suitable claddings on the crystal fiber core are discussed.

Crystal fibers for high power lasers

In this paper, we present our recent progress in developing single crystal fibers for high power single frequency fiber lasers. The optical, spectral and morphological properties as well as the loss and gain measured from these crystal fibers drawn by Laser Heated Pedestal Growth (LHPG) system are also discussed. Results on application of various cladding materials on the crystal core and the methods of fiber end-face polishing are also presented.