Laser heterodyne method for high-resolution gas-density measurements

A new method for noncontact, high-resolution measurement of gas density is described. The method uses a two-frequency Zeeman-split He-Ne laser and cum ulative phase-measuring electronics. The measurement is resolved in two dim ensions and provides density that is averaged only along the length of the laser beam that passes through the test section. The technique is based on highly accurate measurement of the optical path-length change of the laser beam as it passes through a test cell tin principle, to within 0.001 lambda , where lambda is the wavelength of the laser). The technique also provides a very large dynamic range (again, in principle, up to 10(10)), which make s the method additionally attractive. Although the optical path length thro ugh the test section is directly related to the index of refraction, and he nce to the density of the gas, the method can also be used to measure tempe rature (if the gas pressure is known) or pressure (if the temperature is kn own). (C) 2001 Optical Society of America.