Alois Stejskal

Absolute interferometry with a 670-nm external cavity diode laser

In the past few years there has been much interest in use of tunable diode lasers for absolute interferometry. Here we report on use of an external cavity diode laser operating in the visible (lambda approximately 670 nm) for absolute distance measurements. Under laboratory conditions we achieve better than 1-microm standard uncertainty in distance measurements over a range of 5 m, but significantly larger uncertainties will probably be more typical of shop-floor measurements where conditions are far from ideal. We analyze the primary sources of uncertainty limiting the performance of wavelength-sweeping methods for absolute interferometry, and we discuss how errors can be minimized. Many errors are greatly magnified when the wavelength sweeping technique is used; sources of error that are normally relevant only at the nanometer level when standard interferometric techniques are used may be significant here for measurements at the micrometer level.

Using helium as a standard of refractive index: correcting errors in a gas refractometer

The refractive index of helium at atmospheric pressure can be determined from ab initio calculations in combination with careful pressure and temperature measurements. Therefore, helium can serve as a theory-based standard of refractive index; it might be used as a medium of known refractive index for high-accuracy interferometric length measurements or it can be used to characterize and correct errors in a gas refractometer. We have used helium to correct for pressure-induced distortions of two refractometers built by us, where both refractometers basically consist of a laser locked to the transmission maximum of a simple Fabry–Perot cavity. As a proof-of-principle of the helium-correction technique, we have used our device to measure the molar refractivity of nitrogen and we find reasonable agreement with previous measurements. When our two refractometers simultaneously measure the refractive index of a common nitrogen sample, we find that the two systems agree with each other within a few parts in 109.

Wavelength-tracking capabilities of a Fabry-Perot cavity

We have characterized the accuracy of atmospheric wavelength tracking based on a laser servolocked to a simple Fabry-Perot cavity. The motivations are (1) to explore a method for air refractive index measurement and (2) to determine the stability and accuracy of these cavities when employed as a length reference, with potential application to absolute distance interferometry, air-wavelength stabilized lasers, or similar applications. The Fabry-Perot cavity consists of mirrors optically contacted to an ultra-low-expansion spacer with the interior of the cavity open along its length to the surrounding air. Changes in laser frequency are monitored to determine changes in the refractive index of the gas in the cavity. We have studied limitations of this technique that arise from humidity effects, thermal distortion, and (for absolute refractive index measurements where the cavity must be evacuated) pressure-induced distortions. Comparing results from two cavities with very different lengths gives us a very sensitive probe of errors associated with end effects, and pressure-induced distortions can be measured by filling the cavity with helium, whose index of refraction is believed to be well known from ab initio calculations. The uncertainty of refractive index measurements can be greatly reduced when these sources of error are measured and corrected.

Sources of error in absolute distance interferometry

In this paper we describe the status of our research on the use of diode lasers for absolute distance interferometry, and we discuss the major sources of uncertainty that limit the accuracy of this technique for distance measurement. We have primarily employed a 670 nm external cavity diode laser as the tunable source for our interferometer, but currently, we are developing a system based on an 850 nm distributed Bragg reflector laser. These two laser have very different strengths and weaknesses; the primary sources of uncertainty in length measurement depend on which laser is employed.

Frequency stabilization of a green He–Ne laser

A new process for stabilizing the frequency of commercially available 543-nm He-Ne lasers is described. The stabilization method is based on anomalous dispersion of the gain medium. A total of four green lasers have been stabilized-two at the National Institute of Standards and Technology and two at the Institute of Scientific Instruments of the Czech Academy of Sciences-making it possible to study frequency variations of the lasers relative to each other. We have also stabilized a 633-nm laser by the use of the same method used for 543 nm.