Figure 1. Gage block interferometer.

Fringe patterns on a master optical flat.

Figure 3. Fringe patterns on

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in the interferometer: (1) the optical components must be precisely aligned and rigidly mounted and (2) the optical components must be of high quality, i.e. the reference mirror and beam splitter must be plane and the lenses free of aberrations.

Alignment is attained when the entrance aperture, exit aperture and laser beam are on a common axis normal to, and centered on, the reference mirror. This is accomplished with a Gaussian eyepiece, having a vertical and a horizontal crosshair, temporarily mounted in place of the exit aperture, and a vertical and a horizontal crosshair permanently mounted on the reference mirror and intersecting at its center. Through a process of autocollimation with illumination at both entrance aperture and Gaussian eyepiece, the reference mirror is set perpendicular to an axis through the intersections of the two sets of crosshairs and the entrance and exit apertures are set coincident with this axis. In addition, the laser beam is aligned coincident with the axis and the prism adjusted so the laser light coming through the entrance aperture is aligned precisely with the exit aperture. Having an exact 90° angle between the measuring leg and the reference leg is not essential as can be seen from instrument geometry. This angle is governed by the beam splitter mounting. All adjustments are checked regularly to assure their permanence. The gage block and its platen are easily aligned by autocollimation at the time of measurement and no fringes are seen until this is done. Final adjustment is made while observing the fringe pattern, producing the configuration of figure 2.

Overall optical quality is tested by evaluating the fringe pattern produced on a master optical flat mounted on support table P. Photographs showing fringes oriented vertically and horizontally are shown in figure 3. Measurements of the fringe pattern indicate a total distortion of 0.10 fringe vertically in the field. A correction factor proportional to the section of field used can be computed and applied. This correction is relatively small because the field section used in fringe fraction measurement is small. Alternatively, the most likely source of distortion, reference mirror J, can be replaced or reworked, and this course will be taken.

Temperature stability of the interferometer and especially of the gage block being measured is important to precise measurement. For this reason an insulated housing encloses the entire interferometer to reduce the effects of normal cyclic laboratory air temperature changes, radiated heat from associated equipment, operator, and other sources. A box of 3/4 inch plywood with a hinged access door forms a rigid structure which is lined with 1 inch thick foam plastic. Reflective aluminum foil covers the housing both inside and outside.

2.1. Interferometer Corrections

In some interferometers, the entrance and exit apertures are off the optic axis so light falls obliquely on the gage block. Gage block length is then a cosine function of the obliquity angle. In this interferometer

and thus no correction is needed. Periodic interferometer alignment insures that the obliquity angle remains zero.

In most interferometers, the entrance aperture is of finite size, therefore ideal collimation does not occur and oblique incidence effects arise from the resultant of all rays from the aperture area. A laser as used with this interferometer is almost a point source because the aperture is at the common focal point of lens D, and collimating lens E, and at this point the beam diameter is the effective aperture.

In two papers, one by Bruce [3] and one by Thornton [4], the authors developed aperture correction equations which consider obliquity of rays from a finite aperture and fringe intensity distribution. Correction factor 2, for a gage block of length &, measured with wavelength A, using a circular aperture is

for aperture of diameter a, and collimating lens of focal length f. S(v) and C(v) are Fresnel integrals. As aperture diameter aproaches zero, the correction approaches zero. In this interferometer the correction is very small because the laser beam diameter is small at the aperture.

3. THE LASER LIGHT SOURCE

The advantage of a laser light source lies in its unequalled coherence. Conventional spectral light sources have such low coherence that gage blocks longer than 10 inches have in the past been measured by stepping up from shorter blocks. Laser fringe patterns in this interferometer compared with patterns from other spectral lamps are of superior contrast and definition throughout the length range.

The disadvantage stems from the somewhat unstable wavelength of Lamb dip stabilized lasers. Variations can approach 1 part in 107 both short term and long term. This is not serious for most metrological applications but this study required better stability as will be seen later. Commercial Lamb dip stabilized helium neon lasers of nominal .6328 micrometer wavelength were used for most of the measurements described in this paper and their performance was monitored by periodic calibrations. Lamb dip laser calibration and stability has been described by Mielenz, et al [5].

Toward the end of these gage block measurements the newly developed iodine stabilized laser was used. Its stability exceeds that of Lamb dip lasers by at least two orders of magnitude and even exceeds the stability of Krypton 86, the present international length standard. Schweitzer, et al [6] describe this laser and its calibration in detail.

wavelength interferometry requires that the gage block length be known to within +1/4 wavelength (+1/2 fringe) either from its history or from another measurement process. This is no problem for NBS reference blocks.

Laser light must not be allowed to reflect back into its own cavity from interferometer optical components because this will disturb the lasing action and may cause a wavelength shift, wavelength dithering or it may stop the lasing altogether. The reflected light is prevented from re-entering the laser by a polarization isolator consisting of a Glan Thompson prism and a quarter wave plate in the beam as it emerges from the cavity (see B, fig. 1). This assembly is tilted just enough to deflect reflections from its faces to the side of the laser exit aperture.

The isolator linearly polarizes the highly elliptical laser output with the G.T. prism, then circularly polarizes it by phase displacement with the quarter wave plate. Light returning to this assembly from dielectric surfaces in the interferometer is reversed in polarization direction by the 180° phase shift at reflection. Thus, light emerging from the isolator polarized in a clockwise direction returns after reflection polarized counterclockwise. It is then linearly polarized by the quarter wave plate at 90° to the polarization plane of the G.T. prism and is diverted away from the laser cavity by the prism's action.

4. ENVIRONMENTAL CONDITIONS AND THEIR MEASUREMENT

Environmental control of the laboratory holds temperature at 20°+.05°C and water vapor content below 50% relative humidity. Temperature variations within the interferometer housing are attenuated by a factor of about 10 from those in the room thus insuring stability of both interferometer and blocks.

Temperature, atmospheric pressure, and water vapor content of the ambient air in the interferometer light path must be measured at the time the gage block is measured. From these properties the refractive index of the air is calculated and, in turn, the laser wavelength. Gage block temperature is measured so that the length of the block at 20°C, or any other temperature, can be computed.

The measuring system for air and block temperature consists of copperconstantan thermocouples referenced to a platinum resistance thermometer. Figure 4 is a schematic diagram of the system.

A solid copper cube provides a stable reference temperature measured by a Standard Platinum Resistance Thermometer (SPRT) and a Mueller resistance bridge. A well in the cube holds the thermometer stem, the reference thermocouple junction and a liquid of high heat conductivity but low electrical conductivity. There is also a second well whose function will be described later. An enclosed thermocouple selector switch connects the measuring junctions, one at a time, to the reference junction and the

thermal emfs generated by the temperature differences between measuring junction and reference junction are indicated on a nanovoltmeter. The nanovoltmeter is shared with the bridge where it is used as a nulĺmeter. To keep the copper block in a stable state and to minimize thermal emfs in the selector switch, both block and switch are enclosed in an insulated box. The switch is operated through a fiber shaft extending to the exterior and the protruding part of the thermometer is protected by an insulated tube.

Thermocouples generate emfs proportional to the temperature gradient along the wire:

where T1 and T2 are the temperatures of the reference and measuring junctions respectively

is the thermocouple constant.

Minimizing this gradient reduces uncertainties.

Relating the system to the International Practical Temperature Scale of 1968 (IPTS '68) is a three step procedure. First, the SPRT is calibrated using methods described in NBS Monograph 126 [7]. Second, the bridge is calibrated as described in the same Monograph. The third step is the calibration of the thermocouples which is accomplished with a second SPRT and insulated copper cube. The second cube has an extra well for thermocouples being calibrated and is heated one or two degrees to equilibrium. Measured thermal emfs generated by the temperature difference between the two cubes together with the temperatures of the cubes allow computation of a calibration factor for each junction.

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