Simple cryogenic infrared window

A simple, cheap technique is reported that allows materials with both large and small thermal expansion coefficients to be mounted as windows in low temperature cryostats while at the same time avoiding thermal stresses. The construction may be thermally cycled many times with no change in its properties. It can hold differential pressures of at least 1 atm for a 1‐in.‐diam window. This technique seems to be particularly useful for applications where the use of soft materials cannot be avoided (e.g., ZnSe in the infrared)

Resonance‐enhanced low‐pressure optoacoustic cell

A low‐pressure optoacousticcell is described that can be used to lock the emission frequency of a laser. A model is developed which describes the low‐pressure behavior of the optoacousticcell as a function of cell dimensions, gas properties, and operating pressure.Resonantoptoacousticcells are predicted to improve the acoustic signal levels significantly. Experiments were carried out with a cell filled with CF4. The model was found to accurately predict resonator quality, resonance frequency, and acoustic response for pressures ranging from 0.1 to 3.0 kPa. At these low pressures acoustic attenuation processes, slow vibration to translation (VT) relaxation and diffusion to the cell wall strongly influence the acoustic behavior of the cell. Using the relaxation time of the ν4 vibrational mode of CF4 as a fitting parameter its value was determined to be three times slower than VT relaxation from the ν2 level. The experimental values for the response were predicted by the model with an error of less than 10% in the whole pressure range. Predictions for the optoacoustic signal for different resonator dimensions were also confirmed. Model predictions for the optoacoustic signal for mixtures of gases and the influence of the temperature are also given. Especially the option of cooling the gas seems to be attractive for the case of CF4

CF4 combination band absorption spectroscopy

Absorption and linewidth measurements of the R+ (29) (ν2 + ν4) A14 + E9 + F114 transition in CF4 are reported as a function of gas pressure and temperature. From these a value for the Einstein A coefficient (0.0185 ± 0.0010 sec−1) is deduced. A model for low temperature absorption and a detailed analysis of additional absorption lines are given. The Doppler width reported by Eckhardt et al. (J. Mol. Spectrosc.90, 321–326 (1981)) at 150 K is confirmed, while the homogeneous linewidth is found to be 10% smaller. The frequency displacement of the absorption feature from the CO2 9R(12) emission line center is found to be (34 ± 2) MHz independent of temperature and pressure as may be expected. The room temperature absorption measurements of Radziemski et al. (Opt. Lett.3, 241–243 (1978)) at three different pressures are confirmed but the absorption is not due to a single line and therefore the corresponding dipole moment calculated from those data (Proc. Soc. Photo-Opt. Instrum. Eng.288, 209–216 (1981)) is too large

A parametric study of the output of the optically pumped continuous wave CF4 laser

A parametric study of laser output versus CF4 pressure and temperature was performed and correlated with a model for the gain in the system, which includes the relevant relaxation processes. Lasing in CF4 was observed at temperatures below 170 K. Cooling the CF4 gas, the output power of the laser increased from 3 mW at 142 K to 5 mW at 113 K, when 4% of the radiation was coupled out. Chopping the pump, the 16-μm signal consisted of a peak decaying in approximately 2 ms, superimposed on a CW background. This decay is caused by the slow relaxation in the CF4 laser, resulting in filling of the lower laser level. For the CW CF4 laser, vibrational relaxation from the laser lower level is even slower than diffusion to the cold cell walls. To increase the relaxation rate, HD was added. In this molecule, the J=1→3 rotational transition at 447 cm-1 is almost resonant with the ν2 vibration in CF4. Maximum CW output was increased by 25% in a mixture containing 10% HD. At the same time, the lasing pressure range was extende

Diode-pumped 1-kHz high-power Nd:YAG laser with excellent beam quality

The design and operation of a one kilohertz diode pumped all solid-state Nd:YAG master oscillator power amplifier system with a phase conjugate mirror is presented. The setup allows high power scaling without reduction in beam quality

All solid-state diode pumped Nd:YAG MOPA with stimulated Brillouin phase conjugate mirror

At the Nederlands Centrum voor Laser Research (NCLR) a 1 kHz diode-pumped Nd:YAG Master Oscillator Power Amplifier (MOPA) chain with a Stimulated Brillouin Scattering (SBS) Phase Conjugate mirror is designed and operated. A small Brewster angle Nd:YAG slab (2 by 2 by 20 mm) is side pumped with 200 μs diode pulses in a stable oscillator. The oscillator is Q-switched and injection seeded with a commercial diode pumped single frequency CW Nd:YAG laser. The output consists of single-transverse, single-longitudinal mode 25 ns FWHM-pulses at 1064 nm. The oscillator slab is imaged on a square aperture that transmits between 3 and 2 mJ (at 100 and 400 Hz, resp.) The aperture is subsequently imaged four times in the amplifier. The amplifier is a 3 by 6 by 60 mm Brewster angle zig-zag slab, pumped by an 80-bar diode stack with pulses up to 250 μs. After the second pass the light is focused in two consecutive cells containing Freon-113 for wave-front reversal in an oscillator/amplifier-setup with a reflectivity of 60%. The light then passes through the amplifier twice more to produce 20 W (at 400 Hz) of output with near diffraction limited beam quality. To increase the output to 50 W at 1 kHz thermal lensing in the oscillator will be reduced

Thermal compression of atomic hydrogen on helium surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid $^{4}$He surface. The surface gas density is increased locally by thermal compression up to $5.5\times10^{12}$ cm$^{-2}$ at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This, and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.Comment: 20 pages, 11 figure