Anomalous tuning of single mode AlGaAs diode lasers

Deviations from the normal tuning of a TJS AlGaAs diode laser have been observed. In the usual situation, the emitted wavelength of a single mode diode laser as a function of temperature is described by a "staircase" function, that is, the tuning curve consists of regions of smoothly increasing wavelength as a function of temperature separated by positive wavelength mode hops. We have observed a tuning curve for which the regions of smooth temperature tuning are separated by negative wavelength mode hops. It is suggested that this effect is due to the existence of structure in the laser's effective gain curve.     

Frequency equilibration in the vapor-cell atomic clock

Over the past several years, anecdotal evidence has grown indicating that Rb vapor-cell frequency standards exhibit a long "frequency equilibration" period following activation (i.e., time constant /spl sim/10/sup 2/ days). Though this long equilibration period can have important implications for diverse timekeeping systems, the mechanism driving the behavior is not well understood and has been the subject of debate. In this paper, we investigate this phenomenon for a number of Rb vapor-cell clocks with the purpose of 1) establishing the validity of frequency equilibration as a generic vapor-cell atomic clock phenomenon and 2) obtaining characteristics of the equilibration process that can be used to constrain hypothesized frequency-equilibration mechanisms. Consistent with anecdotal evidence, results on three separate manufacturers' clocks indicate that frequency equilibration is a generic vapor-cell clock phenomenon, observable in both laboratory-based and space-based systems. However, the experimental data also casts doubt on the two mechanisms most often offered to explain frequency equilibration: helium permeation through the resonance cell's glass envelope, and the intensity-dependent light-shift effect. To help guide further research, we propose two alternate mechanisms for frequency equilibration: alkali surface diffusion on the resonance-cell and lamp walls, and the spectrum-dependent light-shift effect.     

Precise time synchronization of two Milstar communications satellites without ground intervention

Satellite navigation and communication systems often require precise synchronization among spacecraft clocks. In the traditional method for achieving synchronization, a ground station makes time-offset measurements to the various spacecraft clocks, and then updates the time and frequency of each satellite as needed. Though straightforward in its implementation, disadvantages to the traditional approach include the large workload placed on the ground station, the need for multiple ground stations to view satellites in different geosynchronous positions, and unaccounted-for delays in atmospheric propagation. In early 1996 Milstar became the first satellite system to employ crosslinks for precise satellite time synchronization. At that time, the crystal oscillator clock onboard FLT-1, the first Milstar satellite, had its time and frequency tied (i.e., slaved) to the rubidium (Rb) atomic clock carried onboard FLT-2, the second Milstar satellite. The FLT-2 Rb atomic clock was controlled by the ground, while the slaving of FLT-1 to FLT-2 was accomplished without ground intervention: all timing information required by the slaving algorithm was obtained through the FLT-1 to FLT-2 satellite crosslink. In this paper we will first show the timekeeping capabilities of the two satellite clocks when operating independently, which indicate that both clocks are performing well. Then, we will present ground station measurements of FLT-1 and FLT-2 timekeeping that demonstrate satellite synchronization to better than 150 nsec without ground intervention. As satellites are added to the Milstar constellation, crosslink slaving will minimize ground station timekeeping activities, thereby lowering system operating costs. © 1997 John Wiley & Sons, Ltd.     

Genetic Variants of GPER/GPR30, a Novel Estrogen-Related G Protein Receptor,Are Associated with Human Seminoma

Testicular germ cell tumors (TGCTs) are the most common solid cancers in young men, with an increasing incidence over several years. However, their pathogenesis remains a matter of debate. Some epidemiological data suggest the involvement of both environmental and genetic factors. We reported two distinct effects of estrogens and/or xeno-estrogens on in vitro human seminoma-derived cells proliferation: (1) an antiproliferative effect via a classical estrogen receptor beta-dependent pathway, and (2) a promotive effect via a non-classical membrane G-protein-coupled receptor, GPR30/GPER, which is only overexpressed in seminomas, the most common TGCT. In order to explain this overexpression, we investigated the possible association of polymorphisms in the GPER gene by using allele-specific tetra-primer polymerase chain reaction performed on tissue samples from 150 paraffin-embedded TGCT specimens (131 seminomas, 19 non seminomas). Compared to control population, loss of homozygous ancestral genotype GG in two polymorphisms located in the promoter region of GPER (rs3808350 and rs3808351) was more frequent in seminomas but not in non-seminomas (respectively, OR = 1.960 (1.172–3.277) and 7.000 (2.747–17.840); p < 0.01). These polymorphisms may explain GPER overexpression and represent a genetic factor of susceptibility supporting the contribution of environmental GPER ligands in testicular carcinogenesis.     

A "space experiment" examining the response of a geosynchronous quartz crystal oscillator to various levels of solar activity

Viewing the frequency history of the high-quality quartz crystal oscillator onboard Milstar FLT-1 as a "space experiment," we have examined the response of the crystal to various solar flares that have occurred over the past 4 years. Our results show that, even for the largest solar flares that can be expected, timekeeping onboard a geosynchronous communications satellite need not be unduly perturbed by the enhanced space-radiation environment of a solar flare, so long as the ground station can take mitigating action within a few hours of the flare's onset.     

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 clock's short-term Allan deviation can be measured without reference to another standard in a fully autonomous manner.      

A three-dimensional model of the gas cell atomic frequency standard

Initial calculations from a three-dimensional (3-D) model of the gas cell atomic frequency standard are discussed. In particular, a recent suggestion that the short-term stability of a gas cell standard might be improved by varying the microwave power is considered. Though the authors' results generally support the previous conclusion, they show that the degree of sensitivity is less than that predicted by one-dimensional (1-D) gas-cell frequency-standard model. This difference in predicted sensitivity is a manifestation of the three-dimensional model's more accurate treatment of the clock-cavity microwave field distribution. The more accurate treatment is highlighted by the three-dimensional model's determination of isoefficiency contours (contours showing spatial regions in the clock cavity that have equal efficiency for producing clock signal), and noting their spatial dependence on microwave power.