Crystal jitter

Normally an oscillator circuit Is designed such that the crystal requires a phase shift of 0 degrees to permit work at the series resonance point. Long-and short-term frequency stability are properties of crystal oscillators because very small frequency differences are needed to maintain the phase shift necessary for the oscillation. The frequency stability Is ensured through the quartz crystal, even If there are long-term shifts In the electrical values that are caused by phase jitter due to temperature, ageing or short-term noise. If mass Is added to the crystal. Its electrical properties change. 

Despite such successes, it was obvious that only a continuous wave laser source can do justice to the extremely sharp 1S-2S transition. Production of intense cw radiation near 243 nm remained long an elusive goal. Satisfactory power levels of several mW were first achieved by B. COUILLAUD et al. [19] by summing the frequency of a 351 nm argon laser and a 790 nm dye laser in a crystal of KDP. In the first cw experiment with this source [11], the power in the observation cell was further enhanced with a standing wave build-up cavity. Fig. 3 shows two-photon spectra recorded in this way. Although the resolution is much superior to the earlier pulsed spectra, it remains limited to a few MHz by laser frequency jitter, collision effects, and transit-time broadening. A further at least millionfold improvement in resolution should ultimately be achievable. 

The intrinsic linewidth of the diode laser should be in the 10 KHz range. This differs drastically from experimental observations, where a jitter of 20-200 MHz can occur. The reason for this seems to be optical feedback into the laser cavity by any of the windows between the crystal and the spectroscopic system. Thus in most of the available systems, the periodic vibrations caused by the closed cycle refrigerator, can be seen directly in the jitter of the laser [1,12], (Fig.1.11). 

DNA-based self-assembly appears to be a robust, readily programmable phenomenon. Periodic two-dimensional crystals have been demonstrated for tens of distinct types of DNA tiles, illustrating that in these systems the sticky ends drive the interactions between tiles. Several factors limit immediate applications, however. Unlike high-quality crystals, current DNA tile lattices are often slightly distorted, with the relative position of adjacent tiles jittered by a nanometer and lattice defect rates of 1 percent or more. Some DNA tiles... 

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