1. Field of the Invention
The present invention relates to an air refractometer for measuring a refractive index of various gases.
2. Description of Related Art
Conventionally, a refractometer has been used for measuring refractive index of air. The refractometer has a container with an optical path of a laser beam from a laser interferometer formed therein, where the refractive index of the air is obtained from a difference between a measured value of the laser interferometer when the inside of the container is vacuum and a measured value of the laser interferometer when the air is introduced in the container (xe2x80x9cDevelopment of Air Refractometerxe2x80x9d, Proceedings of JSPE Spring Meeting, 1994, P. 451, 452).
However, since such refractometer alternately creates vacuum and atmospheric condition within a single container, the container etc. can be deformed according to pressure change between the vacuum and atmosphere, so that the accuracy for measuring the refractive index cannot be improved on account of the deformation.
In order to overcome the above disadvantage, another refractometer has been proposed, the refractometer having two laser interferometers, i.e. a laser interferometer using a laser beam advancing in the vacuum and a laser interferometer using a laser beam advancing in the air, thus measuring refractive index of the air from the measured values of the laser interferometers (xe2x80x9cDevelopment of an Air Refractometer and Evaluation of its Performancexe2x80x9d, 1999, 1st International Conference of EUSPEN, P. 145-148).
Specifically, the refractometer uses two laser beams split from a single laser beam. A vacuum container made of metallic bellows is provided to a part of the optical path of the laser beam, the vacuum container having a window on a movable end thereof. One of the laser beams advances through the inside of the vacuum container to be reflected by the window to be incident on one of the laser interferometers. The other laser beam advances through the inside of the vacuum container to be transmitted through the window and further advances through the air to be reflected by a reflector disposed thereahead to be incident on the other laser interferometer. The laser beams are mutually parallel and are of the same advance direction.
Thus arranged refractometer obtains the refractive index of test gas from measured value obtained by measuring a movement of the vacuum optical path caused by moving the movable end of the vacuum container with one of the laser interferometers and by measuring a movement of the air optical path with the other laser interferometer.
However, since the vacuum optical path as a reference standard and the air optical path as a dimension to be measured are disposed in parallel, the refractometer using two laser interferometers does not satisfy Abbe""s principle requiring linear disposition of the reference standard and the workpiece in measurement direction, thus being likely to cause measurement error according to the Abbe""s principle.
The present invention adopts following arrangement to provide an air refractometer capable of reducing the measurement error by satisfying Abbe""s principle, thus improving measurement accuracy.
According to an aspect of the present invention, an air refractometer has: a vacuum container, the length of the vacuum container being variable in longitudinal direction; a vacuum-side laser beam advancing inside the vacuum container in the longitudinal direction; a vacuum-side laser interferometer using the vacuum-side laser beam; a gas-side laser beam parallel to the vacuum-side laser beam and advancing inside a space of a test gas; a gas-side laser interferometer using the gas-side laser beam; a drive mechanism for driving a movable end of the vacuum container along the longitudinal direction; a vacuum-side reflector provided to a vacuum-side of the movable end of the vacuum container for reflecting the vacuum-side laser beam; and a gas-side reflector provided on a test gas side of the movable end for reflecting the gas-side laser beam, a movement of the vacuum-side reflector being measured by the vacuum-side laser interferometer, a movement of the gas-side reflector being measured by the gas-side laser interferometer, the refractive index of the test gas being measured based on the measured values, where the vacuum-side laser beam and the gas-side laser beam are split from a single laser beam and are respectively introduced to the vacuum-side laser interferometer and the gas-side laser interferometer through a single-mode light-waveguides, the respective laser beams irradiated from the respective single-mode light-waveguides being optical measuring light paths between the respective laser interferometers and the respective reflectors, and where the optical measuring light path formed by the vacuum-side laser beam and the optical measuring light path formed by the gas-side laser beam are coaxially located sandwiching the movable end of the vacuum container.
According to the above arrangement, in the two laser beams split from the single laser beam, the vacuum-side laser beam is introduced to the vacuum-side laser interferometer through the single-mode light-waveguide to advance in the inside of the vacuum container, which is reflected by the vacuum-side reflector to be incident on the vacuum-side laser interferometer. On the other hand, the gas-side laser beam is introduced to the gas-side laser interferometer through the single-mode light-waveguide to advance in the space of the test gas, which is reflected by the gas-side reflector to be incident on the gas-side laser interferometer. The movement of the optical measuring light path caused by moving the movable end of the vacuum container between the vacuum-side laser interferometer and the vacuum-side reflector is measured by the vacuum-side laser interferometer. The movement of the optical measuring light path between the gas-side laser interferometer and the gas-side reflector is measured by the gas-side laser interferometer, thus obtaining the refractive index of the test gas based on the measured values.
Here, since the optical measuring light path formed by the vacuum-side laser beam as the reference standard and the optical measuring light path formed by the gas-side laser beam as the dimension to be measured are coaxially positioned sandwiching the movable end of the vacuum container, the Abbe""s principle requiring linear disposition of the reference standard and the workpiece can be satisfied. Accordingly, the measurement error in measuring the air refractive index can be reduced, thus improving measurement accuracy.
In the present invention, the vacuum-side reflector and the gas-side reflector may preferably be provided on either a vacuum side or a gas side of an optical transparent body provided on the movable end, the vacuum-side reflector and the gas-side reflector being formed by a reflective film having a reflective surface of high reflectivity on both sides thereof.
Accordingly, since the reflective film is formed on one side of the optical transparent body, the reflectors can be easily formed on both sides of the movable end, i.e. the vacuum-side and the gas-side, of the vacuum container.
In the present invention, an attitude of the single-mode light-waveguide may preferably be fixed so that an optical coupling efficiency of the vacuum-side laser beam and the gas-side laser beam respectively irradiated from the single-mode light-waveguides is more than or the same as a predetermined value.
Accordingly, since the attitude of the single-mode light-waveguides is fixed so that the optical coupling efficiency of the vacuum-side laser beam and the gas-side laser beam respectively irradiated from the single-mode light-waveguides is more than or the same as a predetermined value, the respective laser beams can be optically located substantially coaxial. In other words, the optical measuring path formed by the vacuum-side laser beam and the optical measuring path formed by the gas-side laser beam can be accurately located on the same axis.
In the above arrangement, an attitude of the reflector may preferably be fixed so that an optical coupling efficiency of the laser beam irradiated from either one of the single-mode light-waveguides and the one of the laser beam reflected by the reflector is more than or the same as a predetermined value.
In the above, while the optical measuring light path formed by the vacuum-side laser beam and the optical measuring light path formed by the gas-side laser beam are located coaxially, the attitude of the reflectors is fixed so that the coupling efficiency between, for instance, the vacuum-side laser beam irradiated from the single-mode light-waveguide and the vacuum-side laser beam reflected by the vacuum-side reflector is more than or the same as a predetermined value. Accordingly, the reflectors can be securely positioned substantially orthogonal with the respective laser beams, thus further improving the measurement accuracy.
In the above, the vacuum-side laser interferometer may preferably measure a frequency of interference fringes by the vacuum-side laser beam and the gas-side laser interferometer may preferably measure a frequency of the interference fringes by the gas-side laser beam, and, while moving the respective reflectors disposed on the movable end at a substantially uniform speed, respective measured values measured by the respective laser interferometers may preferably be multiplied by a frequency multiplying means having the same predetermined multiplying ratio, one of the multiplied measured values being counted as a reference clock of a frequency counter, the other multiplied measured values being counted as a counter clock of the frequency counter, thus obtaining a refractive index of a test gas from the measured values.
Accordingly, since the refractive index of the test gas is measured not by the number of the interference fringes measured by the respective laser interferometers but by the frequency of the interference fringes measured by the respective laser interferometers, the measurement can be conducted with high resolution.
In the present invention, the measured value measured by the vacuum-side laser interferometer may preferably be fed back to a drive controller for controlling the drive mechanism and the uniform movement of the reflector is controlled based on comparison between the feedback information and a predetermined command value of a movement speed, and the drive mechanism may preferably include: a drive body for the movable end fixed thereon; a single drive roller to be rotated in a drive direction of the drive body to drive the drive body while being in contact with the drive body; and a guide mechanism for holding the drive body in a predetermined attitude through a fluid.
Since the uniform movement of the reflectors (movable end of the vacuum container) is controlled by comparing feedback information of the value measured by the vacuum-side laser interferometer fed back to the drive controller and the predetermined command value of the moving speed, the uniform movement of the reflectors can be controlled with high accuracy.
Further, since the drive body is maintained in a predetermined attitude by the guide mechanism holding through a fluid, the drive body can be held in the predetermined attitude without mechanical distortion. Furthermore, since the drive body is driven by rotating the single drive roller while being in contact with the drive body, mechanical constraint of the drive body can be limited to a single portion of the contact portion against the drive roller.
In the above arrangement, the fluid of the guide mechanism may preferably be the test gas.
Since the test gas is used as the fluid of the guide mechanism, the measurement accuracy is not decreased even when the fluid of the guide mechanism and the test gas inside the space for the gas-side laser beam to advance are mixed.
Alternatively, the fluid of the guide mechanism may preferably be air and the guide mechanism includes an exhaust-collecting air bearing.
Accordingly, since the air is used as the fluid of the guide mechanism and the guide mechanism includes the exhaust-collecting air bearing, the fluid of the guide mechanism and the test gas for the gas-side laser beam to advance are not mixed, thus not deteriorating the measurement accuracy.
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