Techniques have been developed to mitigate many of the issues associated with underwater imaging in turbid environments. However, as targets get smaller and better camouflaged, new techniques are needed to enhance system sensitivity. Researchers at NAVAIR have been developing several techniques that use RF modulation to suppress background clutter and enhance target detection. One approach in particular uses modulation to encode a pulse in a synchronous line scan configuration. Previous results have shown this technique to be effective at both forward and backscatter suppression. Nearly a perfect analog to modulated pulse radar, this technique can leverage additional signal processing and pulse encoding schemes to further suppress background clutter, pull signals out of noise, and improve image resolution. Additionally, using a software controlled transmitter, we can exploit this flexibility without the need to change out expensive hardware. Various types of encoding schemes were tested and compared. We report on their comparative effectiveness relative to a more conventional non-coded pulse scheme to suppress background clutter and improved target detection.
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 Ho<sup>3+</sup> 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.<sup>1</sup>
The detection and identification of underwater threats in coastal areas are of interest to the Navy. When identifying a potential target, both two-dimensional (amplitude versus position) and three-dimensional (amplitude and range versus position) information are important. Laser imaging in turbid coastal waters makes this task challenging due to absorption and scattering in both the forward and backward directions. Conventional imaging approaches to suppress scatter rely on a pulsed laser and a range-gated receiver or an intensity-modulated continuous wave laser and a coherent RF receiver. The modulated pulsed laser imaging system is a hybrid of these two approaches and uses RF intensity modulation on a short optical pulse. The result is an imaging system capable of simultaneously acquiring high-contrast images along with high-precision unambiguous ranges. A working modulated pulsed laser line scanner was constructed and tested with a custom-built transmitter, a large-bandwidth optical receiver, and a high-speed digitizing oscilloscope. The effectiveness of the modulation to suppress both backscatter and forward scatter, as applied to both magnitude and range images, is discussed.
Laser imaging through a turbid medium is complicated by scattering. Backscattered photons reduce image contrast as weak target returns compete against a large background of backscattered light. Forward scattering broadens the interrogating laser beam, thereby reducing the spatial resolution of the target. Prior research has shown that intensity modulation (<100 MHz ) can be used to “wash-out” the backscatter, resulting in better discrimination of the target and higher contrast. We show that the higher modulation frequencies (>100 MHz ) can be also used to suppress forward scattered light, thereby increasing spatial resolution.
The detection and identification of underwater threats in coastal areas is of interest to the Navy. Conventional optical imaging systems are limited to scenarios where the number of attenuation lengths between the system and the object are less than 4. With a desire to operate at extended ranges and threats becoming smaller and better camouflaged, new approaches are needed. In response to these challenges, new transmitters and receivers are being developed to support the next-generation of underwater optical imaging systems. One of these systems is based on the modulated pulse concept where a pulsed laser source is encoded with a radar signal, and a range-gated, high-speed optical receiver recovers the radar modulation envelope. Subsequent processing of the radar signal provides a way to discriminate against multiply scattered light and to enhance image contrast and resolution. The challenge is developing transmitter and receiver hardware that meets the requirements of the modulated pulse technique. We report recent progress that has been made in developing modulated pulse transmitter and receiver hardware. A working prototype was demonstrated and tested in a controlled laboratory environment. The results of these initial experiments are presented.
In this paper, we present our recent results in the development of Ho<sup>3+</sup> doped sesquioxides for eye-safe solid state lasers. We have synthesized optical quality Lu<sub>2</sub>O<sub>3</sub> nanopowders doped with concentrations of 0.1, 1.0, 2.0, and 5% Ho<sup>3+</sup>. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nanopowders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.
Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped
oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.<sup>1,2,3</sup> These materials
were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers
to reduce detrimental thermal loading.<sup>4</sup> Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers.
Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped
materials.<sup>5,6</sup> These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”
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.
In this paper, we present our recent results in developing cladded-single crystal fibers for high power single frequency fiber lasers significantly exceeding the capabilities of existing silica fiber based lasers. This fiber laser would not only exploit the advantages of crystals, namely their high temperature stability, high thermal conductivity, superior environmental ruggedness, high propensity for rare earth ion doping and low nonlinearity, but will also provide the benefits from an optical fiber geometry to enable better thermal management thereby enabling the potential for high laser power output in short lengths. Single crystal fiber cores with diameters as small as 35m have been drawn using high purity rare earth doped ceramic or single crystal feed rods by Laser Heated Pedestal Growth (LHPG) process. The mechanical, optical and morphological properties of these fibers have been characterized. The fibers are very flexible and show good overall uniformity. We also measured the optical loss as well as the non-radiative loss of the doped crystal fibers and the results show that the fibers have excellent optical and morphological quality. The gain coefficient of the crystal fiber matches the low quantum defect laser model and it is a good indication of the high quality of the fibers.
First operation of the <sup>4</sup>F <sub>9 / 2</sub> → <sup>6</sup>H <sub>13 / 2</sub> laser transition in dysprosium doped yttrium aluminum garnet is reported.
Room temperature operation at 582.7nm was obtained using 447nm GaN diode lasers pumps. Gaussian single-mode
operation was demonstrated with a non-optimized slope efficiency of 12%. Millisecond pulsed operation generated
150mW with power limited by the pump diodes brightness. An emission cross section of 4.1E-21cm<sup>2</sup> at 582.7nm was
determined by laser threshold analysis.
We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes.
Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the <sup>4</sup>F<sub>9/2</sub> level in trivalent
dysprosium. Modeling based on these measurements suggests this is a promising new laser transition.
Rare earth doped ternary lead salts are being studied for use as mid-IR laser materials. We summarize progress at the Naval Research Labs on the production and evaluation of this important class of solid-state laser.