ABSTRACT
A digital system for controlling the frequency of a continuous wave (CW) ring laser is described. The system utilizes Doppler-free absorption to steer the laser onto a resonance peak within a vapor cell and can hold the laser at this frequency for long periods of time using active feedback. The vapor cell is immersed in a sinusoidally varying magnetic field that produces a feedback signal by exploiting the Zeeman effect so that the laser frequency does not need to be dithered to achieve lock. A bias field can also be applied to adjust the frequency over several MHz while maintaining lock. This is advantageous for cold atom studies that require the laser to be red-detuned from resonance. Signals from the absorption cell and magnetic field are digitized and fed to a dedicated microcontroller that calculates and produces the feedback signal. The digital control system is described, and measurements are presented where the locked frequency is compared to data obtained from a commercial wavemeter. The locked frequency standard deviations ranged from 250 to 450 kHz within a 1 h period. The Allan deviations of the locked frequency had the best stability at a value of 6 × 10-11 in a 10 s averaging period. The locking system described here has application in experiments that require very stable laser frequency control and can also be used for regular recalibration of wavemeter parameters. The design can easily be adapted to control different CW laser systems and hence can be used for a range of atomic targets.
ABSTRACT
The endoplasmic reticulum (ER) is a single organelle in eukaryotic cells that extends throughout the cell and is involved in a large number of cellular functions. Using a combination of fixed and live cells (human MRC5 lung cells) in diffraction limited and super-resolved fluorescence microscopy (STORM) experiments, we determined that the average persistence length of the ER tubules was 3.03 ± 0.24 µm. Removing the branched network junctions from the analysis caused a slight increase in the average persistence length to 4.71 ± 0.14 µm, and provides the tubule's persistence length with a moderate length scale dependence. The average radius of the tubules was 44.1 ± 3.2 nm. The bending rigidity of the ER tubule membranes was found to be 10.9 ± 1.2 kT (17.0 ± 1.3 kT without branch points). We investigated the dynamic behaviour of ER tubules in live cells, and found that the ER tubules behaved like semi-flexible fibres under tension. The majority of the ER tubules experienced equilibrium transverse fluctuations under tension, whereas a minority number of them had active super-diffusive motions driven by motor proteins. Cells thus actively modulate the dynamics of the ER in a well-defined manner, which is expected in turn to impact on its many functions.
