1. Field of the Invention
The present invention relates to a vibration-resistant interferometer which is hard to be influenced by external vibrations as well as turbulence in the air near the sample to be measured. In particular, the present invention relates to a vibration-resistant interferometer which is suitable for measuring the surface shape of optical members and the like, in which the results of measurement should be highly accurate, at their manufacturing sites.
2. Description of the Prior Art
Interferometers have conventionally been known as a means for detecting minute irregularities on the surface of optical members and the like in view of fringes formed by optical interferences.
As high-precision processing technologies have recently been developed, there is a demand for an interferometer which can be mounted on a processing machine or the like so as to measure processing surfaces of products with a high accuracy at their processing sites.
The interferometer mounted on the processing machines or the like at the processing site is easily influenced by vibrations which its optical system receives externally as well as turbulence in the air near the surface to be processed. It is actually difficult for such an interferometer to attain a highly accurate result of measurement unless it can exclude influences of the above-mentioned vibrations and air turbulence to minimize the disorder in interference fringes, even when the interferometer is theoretically supposed to perform measurements with a high accuracy.
As an interferometer which can exclude the above-mentioned influences of vibrations and air turbulence, attention has been paid to a so-called common-path interferometer in which a reference luminous flux and an object luminous flux can travel through nearly the same path so that they are similarly subjected to such influences, thereby substantially canceling their relative phase changes.
FIG. 6 shows a zone-plate interferometer which is a typical example of the common-path interferometer. In this interferometer, in order for a laser light beam 101 to be focused by lenses 102, 103 onto a concave surface, i.e. sample surface 104a, of a sample 104, a zone plate 105 is disposed in the optical path of thus focused luminous flux. As the laser light beam 101 is introduced into the zone plate 105, it is divided into a first luminous flux diverging from the center of curvature of the sample surface 104a and a second luminous flux directed to the center of the surface of the sample 104. Thereafter, these luminous fluxes are reflected by the sample surface 104a and then reenter the zone plate 105. The first luminous flux passes through the zone plate 105 to directly advance, while the second luminous flux is diffracted by the zone plate 105. Accordingly, they overlap with each other and then, by way of mirrors 107, 108, reach the film surface of a camera 109 to form interference fringes thereon which correspond to the irregularities of the sample surface 104a.
The light components of the first and second luminous fluxes reflected on the sample surface 104a respectively function as object light and reference light. Since these two luminous fluxes are reflected on the same sample surface 104a and pass through substantially the same positions, changes in their phase differences will be canceled by each other even if there are external vibrations, air turbulence, and the like. Accordingly, an image of interference fringes obtained by the interference of the two luminous fluxes can yield highly accurate results of measurement.
However, in the common-path interferometer such as the above-mentioned zone-plate (Fresnel zone-plate) interferometer, it is difficult to adjust the positions of the zone plate and other optical systems. Accordingly, its optical systems are complicated in structure and costs for their manufacture and maintenance become higher.