Interferometer drift

Similar arguments can be made for measurements made in the near-infrared region e.g., from 9000 to 4000 cm . In this case, the best performance is usually in the windows between the atmospheric water bands at about 5300 and 7200 cm . Good 200-cm -wide windows for measuring the optimum performance of FT-NIR spectrometers are 4600 to 4400 and 6200 to 6000 cm , while interferometer drift is best examined in the region above 8000 cm ... 

Phase changes as small as 2 tr x 10 rad have been detected 269) by using a null method where phase drifts of the apparatus are compensated by a calibrated and controllable dielectric in cavity B. The response time of this laser interferometer is less than 5 psec when operated at 3.39 pm and repetitively pulsed plasmas can therefore be investigated. Gerardo etal. 270) even reached a time resolution of... 

Even with this gas-pressure tuning interferometer, the drift of t during a long-time measurement is unavoidable. In addition to the mechanical stability of the interferometer, a feedback loop control of t is necessary for such a long-time... 

Optical interferometer techniques can achieve resolution far beyond 1 nm. Because of this high sensitivity of the interferometer sensor, the direct detection of small molectdes at low concentrations should be possible [15]. Detection is generally limited by electronic and mechanical noise, thermal drift, light source instabilities, and chemiced noise. But interferometric devices have an intrinsic reference channel which ofiers the possibility of... 

Infrared analysis of the coated and uncoated alumina particles was done by DRIFT with a 180° backscattering configuration and referenced to powdered (<30-/zm particle size) KC1. A Nicolet 7199 interferometer operating at 4-cm 1 resolution was used to average 250 interferograms for an improved signal-to-noise ratio. The evacuated cell was capable of pressures of 10-6 torr (10-3 Pa) and temperatures to 400 °C. 

Using a double servo loop for fast stabilization of the laser frequency onto the transmission peak of a Fabry-Perot Interferometer (FPI) and a slow loop to stabilize the FPI onto the first derivative of a forbidden narrow calcium transition, Barger et al. constructed an ultrastable cw dye laser with a short-term linewidth of approximately 800 Hz and a long-term drift of less than 2 kHz/h [218]. Stabilities of better than 1 Hz have also been realized [219, 220]. 

A double-beam spectrometer is illustrated in Figure 16-9. The mirrors directing the interferometer beam through the sample and reference cells arc oscillated rapidly compared to the movement of the interferometer mirror so that sample and reference information can be obtained at each mirror position. The double-beam design compensates for source and detector drifts. 

Figure 5.33 Simplified DRIFTS. The sample is mixed with KBr and placed in a cup in a DRIFTS accessory. Light from the interferometer hits the surface at an angle, and the specularly reflected light is blocked while the diffusely reflected light is captured by a curved mirror and directed toward the detector. The sample spectrum is ratioed against KBr. 

The crystal interferometer is a very delicate device air flow around the interferometer causes a serious drift of interference fringes. However, this method can detea the phase shift directly, and the resultant image quality exceeded expeaations, even of radiologists. While X-ray phase imaging was implemented by this two-beam X-ray interferometry for the first time, this approach is still the most sensitive of the X-ray phase imaging techniques. 

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