Yat C. Chan

A Fraunhofer-wavelength magnetooptic atomic filter at 422.7 nm

A demonstration of the first magnetooptic atomic filter that overlays a strong solar Fraunhofer line is reported. Compared with alkali magnetooptic filters, this filter enjoys a large reduction in solar interference and a significant decrease in the number of noise passbands. The filter utilizes the strong Ca(4p/sup 1/P/sub 1/-4s/sup 1/S/sub 0/) transition at 422.7 nm. Under the weak magnetic field experimental conditions, a maximum transmission efficiency of 55% and a symmetrical double-peaked transmission spectrum with 1.5 GHz wide passbands were observed. The filters frequency response was measured with a laser intensity modulation technique. No falloff was observed at 176 MHz, the highest frequency available with the apparatus. Calculations indicate that further improvements in filter performance can be achieved by optimizing the magnetic field and the cell temperature. >

Experimental demonstration of internal wavelength conversion in the magnesium atomic filter.

We describe what is to our knowledge the first experimental demonstration of internal wavelength conversion in the metastable Mg atomic filter. The filter operates at a Fraunhofer line, thereby offering intrinsic solar background rejection. Incoming green Fraunhofer photons (518 nm) are absorbed by metastable Mg atoms in an oven cell and are converted into UV (384-nm) photons. We observed a green-to-UV conversion efficiency of greater than 50%.

Cooling of cesium atomic beam with light from spectrally broadened diode lasers

We have used spectrally broadened counterpropagating radiation from tunable diode lasers to cool an atomic beam of cesium. This produces a continuous beam of cold atoms. The injection current to the single-mode diode laser is modulated at 10 MHz, resulting in spectrally broadened light for atomic cooling and optical pumping. The atomic beam is probed with a weak single-mode laser. This is a simple and relatively inexpensive method for producing a continuous supply of cold atoms.

Passive Fraunhofer-wavelength atomic filter at 422.7 nm

We have demonstrated 25% internal photon conversion efficiency of a new passive atomic filter. The signal wavelength matches an intense Fraunhofer line at 422.7 nm, thereby offering enhanced sunlight rejection. A quasi-molecular interaction promotes rapid energy transfer between the 4p1P1 level and the 4p3PJ level of neutral calcium for wavelength shifting of 422.7-nm light into 657.3-nm emission. We discuss augmentation of the photon conversion efficiency by radiation trapping.

Passive Fraunhofer-wavelength atomic filter at 460.7 nm

A passive Fraunhofer-wavelength atomic filter in the spectral region of minimum seawater attenuation is reported. Signal light at 460.7 nm is strongly absorbed by strontium atoms contained in a heated vapor cell. Collisions with rare gases transfer population into the Sr(5p/sup 3/P/sub j/) level, which subsequently emits 689.3-nm photons. Up to 45% photon conversion efficiency was recorded. The strontium filter exhibits a 50- mu s decay time and a single noise passband. >

Raman lidar calibration for the DMSP SSM/T-2 microwave water vapor sensor

Campaigns were conducted at the Pacific Missile Range Facility, Barking Sands, Kauai, investigating Raman lidar as a method to improve calibration of the DMSP SSM/T-2 microwave water vapor profiling instrument. Lidar mixing ratios were calibrated against AIR and Vaisala radiosondes and the calibration was tested in the vicinity of clouds. Above 6 km, radiosondes reported anomalously low relative humidity in the vicinity of clouds. Lidar measurements were confirmed by using an electro-optical shutter, which provided correct measurement of relative humidity at cloud bases above 6 km. Radiative transfer calculations applied to the lidar data closely matched signals observed in the SSM/T-2 atmospheric channels. Forward calculations for surface sensitive channels disagreed with SSM/T-2 and SSM/I observations. Fine scale surface roughness and localized orographic drying are tentatively suggested as explanations. Cloud effects were ruled out as a significant source of discrepancy.

Self-Monitoring and Self-Assessing Atomic Clocks

From digital communications to satellite navigation, remotely synchronized clocks play a role of primary importance. The failure of these clocks will lead to not only service interruptions, but also, in some cases involving satellite navigation, more dire consequences with potential loss of life. Consequently, ensuring the integrity of remote clocks is now an issue of considerable import. In this paper, we demonstrate that an atomic clock can autonomously assess its own frequency stability and integrity by comparing the phase of its output signal to a delayed version of itself in what is essentially an interferometric technique. Using a high-quality crystal oscillator, we demonstrate that fractional frequency jumps of 10-11 are easily observed and that a cesium atomic clocks short-term Allan deviation can be measured without reference to another standard in a fully autonomous manner.

Tunable infrared filter based on elastic polymer springs

This paper describes the design, fabrication and testing of tunable Fabry-Perot filters. The goal of this research is to develop novel tunable filter with an area of 5x5 mm2 that will be used in infrared gas sensors. This exploits the fact that most gases have unique infrared absorption signatures in the 2-14 µm wavelength region. The filter consists of two thin silicon wafers coated with quarter wave dielectric layers to form wavelength dependent high reflection mirrors and separated by air gaps with an average height of 8, 5.1 and 3.5 μm. The mirrors are supported by four elastic polymer posts (springs) each with an area of 100×100 μm2 made by using photo definable polydimethylsilxane (PDMS). An electrostatic voltage is used to compress the springs, change the airgap height and hence shift the transmission peaks to a shorter wavelength. A finesse of 12 with full width at half maximum (FWHM) of 70 nm, and a peak transmission of 63% were achieved by applying 100 volts on a device with 8 µm post height and wafer thickness of 125 µm. In addition, the measured tunability before and after hard baking of the device was 210 nm and 130 nm respectively. The tunability stayed constant after hard baking the devices and did not show any changes with time. The tunability was also measured on a thinner silicon mirror with 3.5 µm post height. In this case, the filter was tuned 180 nm by applying 10 volts. However, the filter finesse was 3, transmission peak was 40% and FWHM over 200 nm. An antireflection coating was deposited on one side of silicon wafers and a Fabry-Perot filter to study transmission enhancement and satisfactory results were achieved.

Atomic velocity distributions in a hydrogen beam effusing out of a radio frequency discharge dissociator

We have measured atomic hydrogen velocity distributions in an effusive beam coming out of a rf discharge dissociator by using a magnetic deflection technique. Dissociator pressures varied between 0.028 and 0.340 Torr. At low dissociator pressures the measured atomic velocity distributions were narrower than the expected beam Maxwellians; at higher pressures they were indistinguishable from beam Maxwellians at the dissociator wall temperature, indicating full thermalization of the atoms prior to exiting the dissociator. Monte Carlo simulations of the thermalization process within the dissociator reproduce these results, and point out the important role of vibrational excitation of the background hydrogen molecules as an energy loss mechanism. Our results are significant when designing magnetic state selectors for spin‐ or hyperfine‐polarized atomic hydrogen beams.

Injection current calibration of diode laser wavelengths

Abstract When performing precision spectroscopy experiments, it is often necessary to incorporate a stabilized, high finesse Fabry-Perot etalon to provide frequency markers. We show that when the experiments use diode lasers, this technique can be replaced by precision measurements of the laser injection current. We examine the relationship between injection current and optical frequency for three TJS single-mode AlGaAs diode lasers and show that, to the limit of our measurement accuracy, the relationship is linear. By using the hyperfine structure of the D 2 line of cesium as our frequency reference, we are able to determine optical frequencies inside a 9.4 GHz range to within a few MHz; furthermore, we show that we can extrapolate this calibration several GHz beyond the calibration range. This optical frequency-calibration technique is very simple to implement and should be widely applicable.