Interferometer maintaining optical relationship between elements

An interferometer for Fourier transform infrared spectroscopy includes a fixed assembly including a housing, a beam splitter, and a mirror fixedly positioned relative to each other. A movable assembly includes a housing, a mirror, and a motor coil, fixedly positioned relative to each other. A first flat spring has an opening for providing an unobstructed optical path of radiation therethrough. A first end of the first flat spring is secured to the fixed assembly and a second end of the first flat spring is secured to the movable assembly for providing movement of the movable assembly relative to the fixed assembly via the first flat spring. An optical relationship between the beam splitter, the mirror of the fixed assembly, and the mirror of the movable assembly is maintained independent of a distance between the movable assembly and the fixed assembly.

BACKGROUND

The present invention relates to interferometers. It finds particular application in conjunction with maintaining alignment of interferometers and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.

Michelson interferometers are known and have been used in many commercial applications. Also well known is that slight misalignment of the optical elements cause modulation changes that can significantly affect the performance of the interferometer. There have been numerous attempts in the design of commercial interferometers, Michelson interferometers included, to assure that either misalignments have been reduced, or that effects of misalignments have been reduced. Some of these attempts include passive means such as using cube corner mirrors, retro-mirrors, and/or other means to compensate undesirable effects. Others have used active means such as dynamic mirror alignment or active thermal control, among others. Alternatively, readily accessible adjustment mechanisms are made available to the user or maintainer of the interferometer that allows for periodic or necessary reestablishment of the relationships of the optical components to maintain an acceptable alignment condition.

The functioning of a Michelson interferometer is well known, based on the design of Michelson in 1891. Griffiths and deHaseth begin their book “Fourier Transform Infrared Spectroscopy” describing the operation of the Michelson interferometer. It is necessary to have a movable mirror that maintains its perpendicularity and flatness to a wave front while the mirror is moving or has moved to a new position. Any short or long term change (commonly referred to as optical instability) in the perpendicularity or flatness of either the fixed or movable mirrors to the wave front may produce compromised results. Similar results occur if the beam splitter changes flatness or angle relative to the wave front. While the effects of mirror misalignment are described in the Griffiths and deHaseth book, the authors make little attempt to address the specifics of the underlying causes of optical instability or loss of modulation efficiency.

Historically, interferometers have been designed with significant mass and thermal capacity for the purposes of reducing the misalignment effects of mechanical, acoustic, and thermal disturbances. Obviously, instruments using massive interferometers are not easily portable, or even easily movable at best. More recently there have been instruments designed for portability and which are designed to maintain alignment. One such instrument, described by Korb, et al., (Applied Optics, 1 Apr. 1996) and patented by Dybwad (U.S. Pat. No. 5,173,744) is reported to have been used for 3 years without the need for realignment. Unfortunately, the instrument and interferometer described therein requires the use of infrared transmitting prisms that require very stringent manufacturing tolerances, resulting in significant manufacturing costs.

More recently Simon et al. (U.S. Pat. No. 5,309,217) invented an interferometer using a pivot and retro-reflectors. Although the Simon et al. patent states the interferometer is stable, easily aligned, and a compact configuration, the invention presented in that patent requires a plurality of additional mirror surfaces and an increase in associated path length that significantly contributes to optical instability resulting from thermal change. Furthermore, the added optical elements and associated structure certainly challenge the use of the word “compact.”

Flat spring/bearings have been used with interferometers. However, until now, such flat spring/bearings have required periodic realignment when significant temperature changes occur in the interferometer's operating environment. Although the means for realignment is typically achieved by means of an automatic alignment algorithm, actuated through precision stepper motors, the need for a realignment often occurs at inopportune times, which causes significant user inconvenience and/or frustration. In addition, significant costs (both monetary and space) are incurred to effect realignments.

The present invention provides a new and improved compact and portable interferometer system that maintains a condition of substantially permanent alignment without the need for expensive prisms or additional expensive optical elements.

SUMMARY

An interferometer for Fourier transform infrared spectroscopy includes a fixed assembly including a housing, a beam splitter, and a mirror fixedly positioned relative to each other. A movable assembly includes a housing, a mirror, and a motor coil, fixedly positioned relative to each other. A first flat spring has an opening for providing an unobstructed optical path of radiation therethrough. A first end of the first flat spring is secured to the fixed assembly and a second end of the first flat spring is secured to the movable assembly for providing movement of the movable assembly relative to the fixed assembly via the first flat spring. An optical relationship between the beam splitter and the mirror of the movable assembly is maintained independent of a distance between the movable assembly and the fixed assembly.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT

With reference toFIGS. 1 and 2, optical elements, including a beam splitter4, a movable mirror8, and a fixed mirror9, in an interferometer1are illustrated according to a first embodiment of the present invention. Infrared radiation3from a source of infrared radiation2is directed to the beam splitter4, which includes a beam splitting surface5. In one embodiment, the interferometer is used for Fourier transform infrared (FTIR) spectroscopy. The infrared radiation3is split by the beam splitting surface5into a reflected portion6aand a transmitted portion7a. The reflected portion6acontinues on to the movable mirror8, which reflects the reflected portion6aback onto itself as a reflected beam6bthat returns to the beam splitting surface5. In a similar fashion, the transmitted portion7acontinues on to a fixed mirror9, which reflects the transmitted portion7aback onto itself as a reflected beam7bthat returns to beam splitting surface5. The reflected beams6b,7bare recombined at the beam splitting surface5and a portion of the reflected beams6b,7bis reflected as a recombined radiation beam10. The recombined radiation beam10then continues on to a sampling apparatus11. The sampling apparatus11modifies the recombined radiation beam10into a sample encoded radiation12as a function of a sample11ain the sampling apparatus11. The sample encoded radiation12continues on to a detection system13. The sample11ais identified as a function of the encoded radiation12.

When the respective optical distances from the beam splitting surface5to the mirrors8,9are substantially equal, the recombined radiation beam10remains in phase, as there is no optical retardation. If the movable mirror8moves either closer to or further from the beam splitting surface5, but remains perpendicular to the reflected beam6bof the reflected portion6a, a retardation change is created. The retardation change modulates the recombined radiation beam10.

When using the interferometer1as part of a rapid scanning Fourier transform infrared (FTIR) system60, the movable mirror8is typically driven at a constant velocity so as to modulate the recombined radiation beam10in a known way that may subsequently be Fourier transformed to recover frequency information of the recombined radiation beam10and/or the sample encoded infrared radiation12.

If one or more of the optical elements including the beam splitter4, the movable mirror8, and/or the fixed mirror9change(s) position(s), or if the optical relationships change between the beam splitter4, the movable mirror8, and/or the fixed mirror9, some amount of modulation change occurs in the recombined radiation beam10. Such unintended changes in modulation produce unwanted effects and, therefore, are undesirable.

A fixed housing41acts as a structural member of a fixed housing assembly40. The fixed housing assembly40includes the beam splitter4, the fixed mirror9, clamps42a,42b, fasteners43a,43b, beam splitter fasteners44a,44b, and o-rings45. Multiple threaded holes46are provided for affixing the interferometer1to, for example, a frame, instrument housing, or base plate (not shown). A cavity47in the fixed housing41is provided to receive the beam splitter4. Holes48,49provide clearance for the recombined radiation10and the beam splitting radiation portions7a,7b.

The infrared radiation3from the source of infrared radiation2enters a movable housing21of the interferometer1via a through hole defined by a wall20in the movable housing21. The movable housing21is a structural member of a movable assembly generally noted by22, which includes clamp members23a,23b, fasteners24a,24b, the movable mirror8, and a motor coil assembly25. All components of the movable assembly22are rigidly fastened to the movable housing21to form a rigid unit that moves together when actuated by an electromagnetic force26exerted on the motor coil assembly25. The electromagnetic force26is largely exerted either to the left or to the right as illustrated, depending on the polarity of the voltage exerted on the motor coil assembly25. The use of motor coils for driving movable mirrors of interferometers to achieve retardation (e.g., distance) between the fixed and movable assemblies40,22, respectively, is known in the art.

The movable assembly22is further rigidly affixed to flat spring/bearings27a,27bat each end of movable housing21via the clamp members23a,23band the fasteners24a,24b. The two flat spring/bearings27a,27bare substantially the same size, shape, and thickness. Furthermore, when the spring/bearing27ais clamped by the clamp members23a,42aand the fasteners24a,43ato the movable housing21and the fixed housing41, and when the spring/bearing27bis clamped by the clamp members23b,42band the fasteners24b,43bto the movable housing21and the fixed housing41, the spring/bearings27a,27bhave substantially the same unrestricted active spring/bearing length30.

The clamp members23a,23b,42a,42band the fasteners24a,24b,43a,43bact as a means to affix the flat springs/bearings27a,27bto the housings21,41while allowing movement between the movable and fixed assemblies22,40. In alternate embodiments, adhesives, braise solder, welding, epoxy, and extruded metal are used to affix the flat springs/bearings to the housings.

In the illustrated embodiment, the beam splitter4, the mirror9of the fixed assembly40, and the mirror8of the movable assembly22are positioned in a space between planes70,72containing the first and second flat springs27a,27bin their flat condition. The first and second flat springs27a,27brestrict movement of the movable assembly22. Because of the restricted movement, a plane containing the mirror8of the movable assembly22is parallel to all planes containing the mirror8of the movable assembly22at all retardations (e.g., distances) between the fixed and movable assemblies40,22, respectively, of interest so that the optical relationship between the beam splitter4and the mirror8of the movable assembly22remains substantially unchanged except for the retardation. In other words, the optical relationship between the beam splitter4and the mirror8of the movable assembly22is maintained independent of a distance between the movable assembly22and the fixed assembly40.

The optical relationship between the beam splitter4and the mirror8of the movable assembly22is satisfied when an angle of a surface of the beam splitter4is maintained relative to an angle of a surface of the mirror8of the movable assembly22. Similarly, the optical relationship between the beam splitter4and the mirror8of the movable assembly22is satisfied when an angle of an axis of a beam, from the beam splitter4toward the mirror8of the movable assembly22, is maintained at a predetermined angle relative to the surface of the mirror8of the movable assembly22.

As discussed above, the beam splitter4, the mirror9of the fixed assembly40, and the mirror8of the movable assembly22are positioned in the space between planes70,72containing the first and second flat springs27a,27bin their flat condition. However, other embodiments in which one or more of the beam splitter4, the mirror9of the fixed assembly40, and the mirror8of the movable assembly22are not positioned in the space between planes70,72are also contemplated.

FIG. 3illustrates the spring/bearing27a. It is to be understood that the spring/bearing27ais representative of the spring/bearing27b. With reference toFIGS. 1-3, the spring/bearing27aincludes a substantially rectangular opening31for providing unrestricted transmission of the recombined radiation10. The rectangular opening31also provides appropriate modulus characteristics for both bearing the weight of the movable assembly and providing resistance to undesirable shocks, torques, and shears imposed by environmental forces. It is to be understood that changing the location of the source2with the sampling apparatus11and the detection system13is known to those in the art. Although the opening31is rectangular in the illustrated embodiment, it is to be understood that other embodiments including other shapes for the opening of the spring/bearing are also contemplated.

In the illustrated embodiment, the clamped areas32of the spring/bearing27afurther align with the edges of the clamp members23a,42a(23b,42bfor the spring/bearing27b) to define the unrestricted clamping length30. Clearance holes33are provided for the fasteners24a,43a(24b,43bfor the spring/bearing27b), and alignment holes34are provided to accommodate high tolerance pins (not shown) for precise positioning. Although the opening31is rectangular in the illustrated embodiment, it is to be understood that other embodiments are also contemplated in which the opening31is oblong, elliptical, circular, or any other shape that might improve robustness and/or stability.

With reference toFIGS. 2 and 4, the fixed mirror9is positioned in a fixed mirror cavity50. More specifically, a spherical surface portion51of the fixed housing41contacts a spherical surface portion52of the fixed mirror9. Six (6) adjustment screws, one of which is illustrated as54, contact a conical section53of the fixed mirror9. In one embodiment, the screws54are placed approximately symmetrically around a circumference of the conical surface53. In this manner, the screws54are used both to adjust the orientation of the fixed mirror9and, once the adjustment is complete, to rigidly affix the mirror9to the fixed housing41.

During use, the two primary sources of optical misalignment and instability are strains from mechanical and thermal stresses on the interferometer1. In one embodiment, the flat spring/bearings27a,27bare manufactured in matched sets and assembled to the movable and fixed assemblies22,40, respectively, to assure substantially precisely repeatable trajectories and relationships between the mirror8on the movable assembly22and the beam splitter/fixed mirror4,9, which remain precisely fixed relative to each other.

Using at least one pair of the flat springs27a,27bwhose respective planes bound a space62that contains the interferometer optical elements (e.g., the beam splitter4, the movable mirror8, and the fixed mirror9) provides significant symmetry, minimum optical path lengths for the radiation, and minimum structural lengths. These features, along with the proper selection of materials, minimizes the effects of thermal changes in the surrounding spaces. The illustrated design further provides a clear optical path for input or output radiation and a mechanical means for conveniently driving the movable assembly22including the mirror8included therein. Simultaneously, the symmetry and compactness of the flat springs27a,27bminimize strains from thermal and mechanical stresses. Significantly reduced strains results in significantly improved optical stability.

Temperatures change constantly and are typically unpredictable in many instrument operating environments. Interferometers that need to function in such environments typically need to (1) be isolated from the changes, (2) have compensation means to counteract the effects of the changes, and/or (3) be designed in such a way that the effects of such changes are minimized. Historically, interferometers have been designed to be isolated from environmental changes and to counteract the effects of environmental changes. The illustrated embodiment of the present invention helps reduce and/or minimize the effects of environmental changes.

In general, the dimensions of a substance increases as the temperature of the substance itself increases. This relationship is typically stated according to the following formula:
L=Lo(1+A(t−to))whereLois the length of an object at temperature, toA is the coefficient of linear expansion, andL is the length of the object at temperature, t

The coefficient of linear expansion, A, is known and documented for most commonly known materials. In an infrared interferometer, the selection of the beam splitter material determines the useful frequency range of the instrument. One commonly used material, Zinc Selenide (ZnSe) has A=7.2×10−6/C. When using a typical one inch diameter (25.4 millimeters) beam splitter, there is approximately 1.5 microns of diameter change for each ten degree change on the centigrade scale if the diameter is unrestricted. While this change appears relatively small, it is capable of creating enormous stresses if the beam splitter were constrained. For example, for a ZnSe beam splitter that is securely affixed to an aluminum housing (a very common practice in commercial FTIR's) the aluminum would attempt to change at approximately three times the rate of the ZnSe. The resulting stress, if not properly dissipated, could result in surface distortion and/or angular (e.g., tilt) change that could result in modulation change and instability.

In one embodiment, the fixed housing41, the movable housing21, the first and second flat springs27a,27b, the clamp members23a,23b,42a,42b, the fasteners24a,24b,43a,43b, and the mirrors8,9are steel. In addition, it is contemplated that the mirrors8,9include a metalized film for improved reflectivity in the infrared range.

Steel and titanium have coefficients of thermal expansion much closer to that of ZnSe. Therefore, using steel or titanium in place of aluminum for the housing would reduce the strain differential between the ZnSe and the housing. However, the benefit of improving strain differential by changing from aluminum to steel or titanium to overcome other factors such as shape, thermal conductivity, thermal absorption, and thermal emissivity, among others, has not been previously demonstrated. Presumably, the improved strain differential has alone not achieved improved results because other factors have been a major source of optical instability. In that regard, our invention strongly suggests that the roles of shape and symmetry are of equal, if not greater, importance than the role of differential coefficients of thermal expansion.

The illustrated embodiment of the present invention has established that combining highly symmetrical, minimum volume shapes and corresponding minimum path lengths for the interferometer reflected and transmitted portions6a,7a, respectively, and the reflected beams6b,7bhas resulted in highly stable modulation over a wide temperature range. Minimizing differences between the coefficients of thermal expansion further improves optical stability.

With reference again toFIG. 2, the illustrated embodiment shows the interferometer1is symmetrical about a plane64, which is located midway between the sides of the interferometer1and the plane of the sheet of paper on which the section shown inFIG. 2is illustrated. Thermal changes to the flat spring/bearings27a,27bare substantially identical since the surface areas of the springs are substantially identical and are exposed to convection in much the same manner. The structural differences between optical components are held to a minimum, determined by the highest resolution possible for the interferometer, and for the beam splitter size being used.