Tim G. Haskell

Growth of first-year landfast Antarctic sea ice determined from winter temperature measurements

Abstract Temperature profiles of first-year landfast sea ice have been recorded continuously over the 2003 winter growth season at McMurdo Sound, Antarctica. The temperature gradients in the ice were used to calculate the growth rate due to conductive heat flux, which is shown to account for only part of the total ice growth. Remaining ice growth must be due to a negative oceanic heat flux. Significantly, this oceanic heat flux is shown to occur episodically, sometimes with sustained daily rates in excess of –30Wm–2. There is no direct correlation between oceanic heat flux and water temperature. Times of increased oceanic heat flux do coincide with the appearance of platelet ice in cores, and appear to account for the growth of 35% of the total platelet ice depth measured in ice cores.

Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl methacrylate co-polymers

Abstract The laser performance of a range of solid state dye lasers based on rhodamine 590-doped co-polymers of 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) has been investigated. The optimisation of preparation conditions, including polymerisation initiator and solvent for dye delivery is discussed in detail. Laser efficiency is compared for different polymeric hosts and dye concentrations with a range of output couplers, cavity lengths and repetition rates. Passive and dynamic loss have been determined for each host medium. Laser efficiencies of optimised polymers are among the highest reported for rhodamine 590-doped solid state dye lasers under these operating conditions. Highest slope efficiency of 35% and lowest threshold fluence of 0.06 J cm −2 were obtained with dimethyl sulphoxide (DMSO) additive in MPMMA at 10 Hz repetition rate.

Real-time restoration of a blurred image with a liquid-crystal adaptive-optics system based on all-optical feedback interferometry

Abstract Real-time restoration of a blurred image using an all-optical adaptive-optics system with a liquid-crystal (LC) device is described. This system is based on a feedback interferometer, where the two-dimensional output fringe intensity from a Mach–Zehnder interferometer with large radial shear is optically fed back to an optically addressed phase-only LC spatial light modulator (SLM). In operation the phase of the reflection-type modulator approximates to half of the conjugate of the input aberrated wave front without a separate aberration-free reference wave. We applied this system to a problem of imaging through a phase-distorting medium and observed that an image blurred by the phase disturbance was restored immediately after the feedback loop was closed. In this case the response time and the resolution of the SLM were about 60 ms and 40 line pairs/mm, respectively.

Formation and structure of refrozen cracks in land‐fast first‐year sea ice

[1]xa0This study characterizes the healing process and structure of undeformed, linear, parallel-sided, flooded cracks in land-fast sea ice. Field investigations and refreezing experiments were performed in McMurdo Sound, Ross Sea, Antarctica, between 1998 and 2002. Data from a two-dimensional thermistor array are used to show that the ice-water interface of freezing cracks is arch-shaped due to bidirectional heat flow to the surrounding ice cover and to the atmosphere. Ice growing laterally into the crack is found to desalinate over a prolonged period of time, until the isotherms are approximately horizontal. Superposition of heat flow to the atmosphere and to the host sea ice sheet allows the refreezing progress to be modeled analytically. Close to the ice–air interface, the salinity is higher at the sides of wide refrozen cracks than it is at the center. However, deeper down and in narrower cracks in general, the salinity is higher at the center than at the sides. A finite volume, computational fluid dynamics (CFD) model reproduces the generally arch-shaped alignment of brine pockets. This pattern is attributed to convection in the mushy layer. Crystals are found to grow upstream into the crack due to a salinity gradient in the buoyant convective flow.

Characterization of dye-doped media for real-time holography

Abstract Studies of saturable dyes used as holographic media where saturation gives rise to both amplitude and phase holograms are presented. Using a simple absorption model together with the Kramers-Kronig relation, a theory for the prediction of holographic diffraction efficiencies over a range of read-wavelengths was developed. Results of experimental tests are presented as support for the model.

Efficient, high photostability, high brightness, co-polymer solid state dye lasers

Abstract We report an investigation of the laser beam quality, efficiency and photostability of rhodamine 590 solid state dye lasers with hosts based upon 2-hydroxyethylmethacrylate and methylmethacrylate P(HEMA:MMA) co-polymers and modified methylmethacrylate (MPMMA) polymers with ethanol and dimethylsulphoxide additives. Highest efficiencies were obtained with MPMMA, with slope efficiencies of up to 44% (corresponding to 61% optical-optical conversion) measured for ethanolic MPMMA. P(HEMA:MMA) hosts gave superior photostability, with the highest normalized photostability of 240 GJ mol−1 obtained for a P(HEMA:MMA) 1:1 rod. A simple resonator has been used to compensate the effects of thermal lensing and pump mode-mismatch, resulting in optimized beam quality for MPMMA lasers.

Highly efficient and photostable perylene orange doped polymer and co-polymer lasers

Summary form only given. Solid-slate dye lasers (SSDLs) offer a simple, efficient and cost-effective means of providing wavelength agility from frequency-doubled and tripled Nd lasers. However, significant improvements must be made in matching new host media to existing dyes and to emerging dyes such as the pyrromethene and perylene series. Of particular interest are new copolymer hosts, which can provide enhanced polymer cross-linking and broad dye compatibility, and the dye perylene orange, which has recently been shown to operate with very high efficiency in a porous glass host.

Efficient narrowband distributed feedback dye-doped polymer laser with a dynamic grating

Summary form only given. We report, to our knowledge, the first tunable solid state DFB laser using a dynamic, pump-induced grating. We used Perylene Red dye-doped polymer DFB gain media, fabricated by dissolving Perylene Red laser dye in methyl methacrylate monomer prior to polymerisation. The dye-doped poly(methyl methacrylate) (PMMA) polymers were cut into trapezoidal slabs with the output faces slanted at /spl sim/10/spl deg/ to eliminate parasitic lasing due to Fresnel reflections from these faces. The pump laser was a second harmonic single mode Q-switched Nd:YAG laser.

Optimized fabrication and laser performance of copolymer solid-state dye lasers

We report pulsed laser operation of rhodamine 530 dye in conventional MPMMA, and in a range of P(HEMA:MMA) copolymer hosts comprising methyl-methacrylate and 2-hydroxyethyl-methacrylate monomers. Samples of high optical quality have been fabricated.