Alignment mechanism for photoelastic modulators

A mechanism and method for precisely arranging two or more optical elements, such as those incorporated into photoelastic modulators (PEMs), at a specific angular orientation. The method includes supporting one optical element in an annular mounting member that has an optic axis, and supporting other optical elements in other annular mounting members that have optic axes, and concentrically stacking together the two or more mounting members about a central axis in a manner such that one mounting member may be rotated relative to the others about the central axis and such that the optic axes of the mounting members define an optics angle, and rotating one mounting member relative to the others to define the specific angular orientation of the optical elements.

TECHNICAL FIELD

This application relates to a mechanism and method for precisely arranging the optical axes of two or more optical elements, such as those incorporated into photoelastic modulators, in a selected angular orientation.

BACKGROUND

A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term “photoelastic effect” means that an optical element that is mechanically stressed and strained (deformed) exhibits birefringence that is proportional to the amount of deformation induced into the element. Birefringence means that the refractive index of the optical element is different for different components of a beam of polarized light.

A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element. The transducer vibrates at a fixed frequency within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended along the axis of the optical element as a result of the vibration. The combination of the optical element and the attached transducer may be referred to as an optical assembly. The axis about which the optical element vibrates is referred to as the optical axis of the PEM.

The optical assembly is mounted within a housing or enclosure that normally includes an aperture through which the light under study is directed through the optical element in a direction generally perpendicular to the optical axis of the PEM. The enclosure supports the optical assembly in a manner that permits the optical element to be driven (vibrated) within it to achieve the above-noted photoelastic effect.

PEMs are commonly used in measuring polarization properties of either a light beam or a sample. Many instruments use two or more PEMs to provide measurements of certain polarization properties. When two PEMs are used in a single instrument, they are typically arranged so that their optical axes are oriented to be precisely 45 degrees apart (as considered in a direction perpendicular to those two optical axes).

Examples of typical, two-PEM instruments include complete Stokes polarimeters, Tokomak polarimeters, and a number of other polarimeters and ellipsometers. When four PEMs are used in one instrument, the PEMs are typically grouped in separate pairs.

The speed and precision with which a pair of PEMs can be oriented so that their optical axes are fixed at a particular, selected angle depends greatly on the precision with which the housing or enclosure to which the PEMs are mounted can be positioned and secured to place the PEMs in that proper orientation.

SUMMARY OF THE INVENTION

The present invention is directed to a mechanism and method for precisely arranging two or more optical elements, such as those incorporated into PEMs, at a specific angular orientation.

DETAILED DESCRIPTION

One embodiment of a mechanism20formed in accordance with the present invention is depicted in the figures. The mechanism20includes two generally annular mounting members, hereafter referred to as a lower mounting member22and upper mounting member24. The designation of “lower” and “upper” is for reference purposes only. The mounting members22,24are nearly identical in construction and are interchangeable. The following description focuses on the upper mounting member24with the understanding that the lower mounting member22is similarly constructed except where otherwise specified.

The upper mounting member24is metal and is generally annular with a depth (measured vertically inFIG. 2) that is about one-half of its radius (as measured in a plan view). A notch26(FIG. 1) is cut through the mounting member24. An open end of a somewhat elongated enclosure28is attached to the mounting member at the notch26to protrude radially outwardly therefrom. (The corresponding enclosure of the lower mounting member22is shown at128.) It is noteworthy that the mounting member may also be formed of rigid plastic, such as Delrin®. This would be a useful configuration when the mechanism is used in a magnetic field as occurs, for example, in Tokomak polarimeter applications.

A primary function of each mounting member22,24is to support the optical assembly of a photoelastic modulator (PEM)30. The primary components of the PEM's optical assembly include an optical element32formed of fused silica. Other material, such as fused quartz, calcium fluoride, zinc selenide, silicon and others may be used to form the optical element. (The corresponding optical element supported in the lower mounting member22is shown at132.)

The optical element32is a generally square-shaped member but having beveled corners that define flat support surfaces34, the function of which is described below. The optical element also has an entry surface36against which an incident light beam is directed while the PEM is operating. A quartz piezoelectric transducer38(FIG. 1) is bonded to one of the four sides of the optical element32. Electrical leads (not shown) from the transducer are connected to a driver circuit for vibrating the optical element32.

The optical element32is supported so that its entry surface36extends across the central aperture40of the upper mounting member24. Preferably, the center of the entry surface is aligned with the central axis41of that aperture40(FIGS. 2 and 3). The optical element32is free to vibrate when driven as described above. In this regard, the optical element32is mounted to the upper mounting member24by somewhat flexible supports42(FIG. 1) that secure the optical element32at each support surface34so that the optical element is substantially suspended within the central aperture40of the upper mounting member24.

Each one of the supports42includes an elastomeric rod44that may be formed, for example, from extruded silicone (polysiloxane) cords that are cut to a specified length to define the rod44. One of the two, flat ends of the rod44is attached, as by an adhesive, to one of the support surfaces34on the optical element32.

The other, free end of the rod44fits within a sleeve46that is carried inside of a cylindrical slider48. The sleeve46has a cylindrical axial bore formed through one end to receive the elastomeric rod44. The sleeve46is a rigid, externally threaded member that is threaded into an internally threaded bore50(FIG. 3) of the slider48.

Each on of the four sliders48fits inside of a radial hole52(FIG. 3) formed through the curved side56of the upper mounting member24. The slider48, with the sleeve46threaded into its bore50, is slid with the radial hole52until the rod44is received in the bore of the sleeve46. The slider48is secured in place via a setscrew54that is threaded vertically (FIG. 1) through the upper mounting member24to bear against the slider.

With the slider secured in place, the sleeve46is advanced until the free end of the rod44(that is, the end not bonded to the optical element support surface34) is completely received within the bore of the sleeve. The sleeve46may be advanced by hand or with a tool. In this regard, the outer end of the sleeve46may be shaped to define a socket for an Allen-type wrench or the like that can be extended into the bore50of the slider to reach the socket in the sleeve46.

The foregoing description of an exemplary support42applies to all four supports42on both mounting members22,24. As depicted inFIG. 1, four supports are employed to secure the optical element32in place relative to the upper mounting member24. The supports are thus arranged in diametrically opposed pairs. Alternative configurations of such supports42are contemplated, such as those described in U.S. Pat. No. 7,800,845 owned by the assignee of this application. As another alternative, the rod44could be replaced with a glass or plastic conical member with the base of the cone bonded to the support surface34and the pointed end seated in the central opening of an annular elastomeric grommet that is mounted on the end of a cylindrical barrel that is secured in the hole52. In the figures, the grommet and barrel would appear as the sleeve46and slider48respectively. The setscrew54would hold the barrel and grommet combination in place.

The transducer38is attached to the optical element32, and not otherwise supported by the upper mounting member24. The transducer38is an elongated, bar-like member that extends from the optical element32and into the enclosure28that protrudes radially outwardly from the outer, curved surface56of the upper mounting member24. The longitudinal axis58of the transducer38is aligned with the center of the optical element32and, as such, this axis58coincides with the optical axis of that optical element.

For purposes of this description, the projection of the optical axis of the optical element32of the PEM30onto the structure of the upper mounting member24is illustrated by axis line58, which will hereafter be referred to as the optics axis58of the upper mounting member24. The lower mounting member22has a similarly defined optic axis158, as shown inFIGS. 1,3and4.

The angle between these two optics axes58,158(as viewed along the central axis41(seeFIGS. 1 and 4) is referred to as the optics angle60. It will be appreciated that the optics angle60(and the associated adjustments to that angle discussed below) corresponds directly to the angle between the optical axis of the optical element32in the upper mounting member24and the optical axis of the optical element132in the lower mounting member22. Any minor variations between those two axes (which may be attributable to, for example, a slight misalignment of the supports42that secure the optical elements32,132in place) can be accounted for as will be discussed below.

As best shown inFIGS. 1 and 2, each mounting member22,24, includes three, spaced-apart, elongated guide slots62,162that, in plan view, are curved about the central axis41. The guide slots62,162are counterbored into the opposite flat surfaces of the upper and lower mounting members22,24. The counterbored portions64,164of the guide slots thus provide recesses within which the opposite ends of shoulder bolts66are received.

As shown in the figures, the upper mounting member24and lower mounting member22are stacked together, concentric with the central axis41. The guide slots62,162are precisely, concentrically aligned so that the smooth, shoulder portion68of each shoulder bolt66fits vertically through the stacked mounting members (SeeFIG. 2) to serve as guide pins so that one mounting member can be precisely rotated relative to the other about the central axis41. The head of each shoulder bolt66fits inside a counterbored portion64. The threaded end of the bolt, to which a flanged nut70is fastened, also fits inside of a counterbored portion164of the guide slots. The nuts70are sized so that they will not rotate with the bolt66. When the precise, desired optics angle60is established, the shoulder bolts66are tightened (as by an Allen wrench applied to the hex socket in the bolt head) to lock the upper mounting member24to the lower mounting member22, thereby fixing the optics angle.

In a preferred embodiment, the upper and lower flat surfaces of the stacked upper mounting member24and lower mounting member22are provided with thin cover plates72, the uppermost plate being added after the bolts66are all tightened. The underside of the radially protruding portion of the enclosure28of the upper mounting member24has a cover plate73, and the upper side of the radially protruding portion of the enclosure128of the lower mounting member22has a cover plate75(FIG. 3).

It is contemplated that once the upper and lower mounting members24,22are stacked but not rotatably fixed together by bolts66, any one of a variety of actuators may be employed for precisely rotating one mounting member relative to the other until the desired optics angle60is established. The actuator may be applied to any part of one mounting member to force rotation of that mounting member relative to the other. The actuator can be connected to a work surface adjacent to the rotated mounting member. Alternatively, the actuator can be connected to one mounting member (which member is secured to be stationary) and operable to apply force to the other mounting member. The actuator may be a permanent component of the overall mechanism, or be configured for removal once the precise optics angle is established, and the mounting members locked together. The actuator can be manually operated or mechanically driven under computer control.

In a preferred embodiment, an actuator74(FIG. 1) for providing precise rotation of one mounting member relative to the other comprises a fine adjustment screw assembly76. That assembly includes and elongated screw78, one end of which80is rounded and engages an exterior surface of the enclosure28that protrudes radially from the upper mounting member24(FIG. 1). The screw78is threaded through a bushing82that is mounted within a base84of the assembly. The base84(hence the assembly) is connected to the cover plate75of the lower mounting member enclosure128via fasteners that are threaded into mounting holes86(FIG. 4). Rotation of the ultra fine pitched screw is transferred via the contact of end80with the enclosure28into rotation of the upper mounting member24relative to the lower mounting member22, which member22may or may not be secured in place while this adjustment is made.

As noted above, the angle between the two optics axes58,158(namely; the optics angle60) that is adjusted as just described corresponds directly to the angle between the optical axes of the optical elements32,132in the respective upper mounting member24and lower mounting member22. Any minor variations between one optic axis58,158and the corresponding optical axis of the associated optical elements32,132(which variations may be attributable to, for example, a slight misalignment of the supports42securing the optical elements32,132in place) can be addressed while the mechanism20is located in an optical setup with light passing though the optical elements of both PEMs and detected. This approach can be referred to as the PEMs optical angle calibration.

One approach to this calibration is schematically depicted inFIG. 2, where the mechanism20is part of a setup that includes a light source86, adjustable polarizer88, an adjustable analyzer90, photodetector92and an associated lock-in amplifier94. The procedure discussed next is for precisely establishing the angle between the optical axes of the two PEMs to be 45°, although other angles may be selected.

The polarizer88is set at 0° and the analyzer90is set at 45°. The upper mounting member24is rotated as described above until the optics axis60is at 45°. This angle can be measured in any of a number of ways, including the use of angular graduations on the exposed, adjacent surfaces of the mounting members. Next, the PEM30in the upper mounting member24is operated at a peak retardation of one-half wave while the PEM in the lower mounting member22remains off. The 2F signal on the detector92is monitored using the lock-in amplifier94. The mechanism20is then employed to precisely rotate the upper mounting member24relative to the lower mounting member22until the 2F signal reads “0,” at which point the upper mounting member24and lower mounting member22are locked together using the shoulder bolts66as described above.

While the foregoing description was made in the context of a preferred embodiment, it is contemplated that modifications to the embodiment may be made without departure from the invention as claimed. For example, it is contemplated that the preferred embodiment of the actuator74may include the application of a spring or latch member extending between the adjustment screw and enclosure28so that the enclosure will move with both the extension and retraction of the adjustment screw78. Further, the actuator may be configured to act on any portion of the mounting members to impart the relative rotation, such portions can be considered protrusions but need not be the radially protruding enclosures discussed above.