| 摘要 |
<p>PURPOSE:To simplify the constitution of an optical system and to improve high heat stability by incorporating an optical system where a reference beam and a measurement beam pass through optically the same path and compensating variation with temperature. CONSTITUTION:The optical system 20 consists of a polarization beam splitter 23 which has a beam splitting surface 24 at 45 deg. to incident light, lambda/4 plates 25 and 27, and a cube corner 29. The lambda/4 plates 25 and 27 are made of the same materials to the same thickness and the lambda/4 plate 27 has one surface coated with a film having a high reflection factor. Here, the path (a) of the reference beam 13 which passes through the splitter 23 and lambda/4 plate 27 is as long as the path a' of the measurement beam 15 which passes through the splitter 23 and lambda/4 plate 25. Similarly, the path (b) of the beam 13 is as long as the path b' of the beam 15. Further, the return path (c) from the splitting surface 24 to a corner 29 is common between both beams 13 and 15. Therefore, if the temperature of the optical system 20 varies to affect the size and refractive index of an optical element, the optical length of the beams 13 and 15 is affected similarly when the elements of the optical system 20 are still in a thermally balanced state.</p> |
