DUAL BEAM FOURIER SPECTROMETER

Abstract: Dual beam Fourier-type spectrometer incorporating distinct sample (11) and reference (13) beams which enter the Michelson-type interferometer on the same side of the beamsplitter (17) and propagate at slightly different angles through the interferometer. The portions of both beams which emerge from the side of the beamsplitter (17) opposite the input side are directed to two separate optical detectors (33 and 35) the outputs of which are electronically subtracted. In a preferred embodiment, the interferometer is of the refractively scanned type, wherein the increased field-of-view facilitates the use of beams having different propagation angles.

Description: DUAL BEAM FOURIER SPECTROMETER Background of the Invention This invention relates to spectroscopy and particu¬ larly to Fourier transform spectroscopy, in which a Michelson interferometer is generally used. The pattern traced out, as the length of one interferometer arm is scanned, is the Fourier transform of the wavelength . spectrum. This invention more specifically relates to a "dual beam" Fourier spectrometer. For the present discussion, the term, ."dual beam" will refer to an .optical system incorporating a Michelson interferometer and having two distinct optical paths in either the input or output of the interferometer (or both) . In a dual beam Fourier spectrometer, these paths are used to simultaneously obtain (a) data from a material sample under study and -(b) data for reference purposes from a sample-free region. This use of the term dual beam should be distinguished from the use of the same term to refer to the fact that the Michelson interferometer is itself a "dual beam" sys¬ tem, i.e., an interferometer in which light is separated by the beamsplitter into two beams and then recombined. "Dual beam" spectroscopy is the subject of Chapter 7 of the book "Chemical Infrared Fourier Transform Speσtro- scopy" by Griffiths, published by John Wiley & Sons. The importance of dual beam Fourier spectroscopy (FS) stems from the fact that the interferogram corresponding to a broad radiation spectrum will have a very high peak- value (central maximum) when the path lengths of the two interferometer arms are equal (see Griffiths, Fig. 1.11). In order to electronically perform the Fourier transforma¬ tion required to obtain the spectrum, it is first necessary to digitize the interferogram, and the cost of Analog-to- Digital (A/D) converters rises rapidly with increased According to Griffiths, the system described above is probably the preferred .approach to dual, beam Fourier spec- trome ry. However, it has- the disadvantage of requiring large area retro-reflectors and a beamsplitter with high optical quality over the full aperture. And, needless-to- say, precise control of -the motion of a large retro-reflec¬ tor is difficult. In addition, the fact that the two beams emerge from opposite sides of the beamsplitter means that they will be affected in different ways by the properties of-the beamsplitter. Of particular significance is the effect of the second (low reflectance) surface of the beam¬ splitter. The resultant imperfect matching of the two beams can be quite significant. This problem is recognized by Griffiths (see pages 177 and 183) who states that it may, in part, be caused by oxides of germanium on the upper surface of the beamsplitter (page 177) . In addition to the design described above, Griffiths discusses several other dual beam spectrometers, some of which have the separated beams in the source portion of the system, rather than the detector portion. All of these systems share the common feature that the two beams either emerge from or enter opposite sides of the beamsplitter. They thus share the beamsplitter asymmetry problem men¬ tioned above. Summary of the Invention The present invention eliminates the problem of imper¬ fect beam matching by having both beams of the dual beam spectroscope emerge (and reach the radiation detecting means) on the same side of the beamsplitter, preferably the side of the beamsplitter opposite the entering radiation. It is desirable that the radiation source propagate the dual beams through the interferometer at slightly different angles. In a preferred version of the present invention, the concepts just discussed are combined with a "refractively scanned interferometer," of the type first disclosed in__my_ prior application Serial No. 790,457, filed April 25, 1977 Because the refractive scanning concept solves certain- problems inherent in the use of dual beam spectrometers, this combination provides important synergistic benefits. Brief Description of the Drawings Figure 1 is a diagrammatic showing of a spectrometer wherein the optical, or interferometer, portion illustrate my improved dual beam system; Figure 2 is a diagrammatic showing of the optical por tion of the preferred system utilizing refractive scanning ' and Figure 3 is a diagrammatic showing of means for ob¬ taining the optimum arrangement of radiation in a dual bea system, wherein the two beams have maximized overlap at th beamsplitter. Detailed Description of Specific Embodiments Figure 1 illustrates one embodiment of my invention. Radiation from two adjacent sources 11 and 13, or from adjacent areas on a single source, is collimated by means of lens 15, o other suitable optical means, and is direct¬ ed to beamsplitter 17 of a Michelson interferometer. Rad¬ iation from source 11 is represented in the figure by solid lines, and radiation from source 13 is represented by dashed lines. After being split and reflected by fixed and moving mirrors 19 and 21, the radiation is recombined at the beamsplitter. The portion emerging from the side of the beamsplitter opposite the input side is imaged by optical means 23. The term "input side" refers to the side of the beamsplitter where the radiation source is located. Since the two radiation source areas 11 and 13 are spatially distinct, the corresponding radiation beams will propagate at slightly different angles through the inter¬ ferometer and will be imaged in separate adjacent areas. The radiation in these two image areas is re-directed by mirrors 25 and 27 and imaged by optical means 29 and 31 on detectors 33 and 35. The outputs of the two detectors are fed into a difference amplifier 37. If a sample to be analyzed 39 is interposed in one of the spectrometer beams, an interferogram will appear at the ■output of the difference amplifier. This can be processed by a Fourier transform computer 41 and fed to a suitable display means 43 to give absorption spectrum of the sample. A significant distinction between the system of Figure 1 and the previous dual beam Fourier spectrometer systems is that the portions of the radiation corresponding to the reference and sample beams, respectively, follow paths which enter the interferometer on the same side of the beamsplitter and which emerge on the opposite side of the beamsplitter. As a result, they are affected in approxi¬ mately the same way by the beamsplitter characteristics. Unlike the earlier systems, my design gives rise to sample .and reference interferograms having the same polarity. In -order to obtain cancellation, it is thus necessary to take the difference between the signals obtained from the two detectors rather than the sum. A preferred embodiment of the optical portion of my invention is shown in Figure 2. This embodiment uses the angular beam differentiation approach of Figure 1 combined with a refractively scanned interferometer such as that described in my earlier patent application (Serial No. 790,457). The embodiment in Figure 2 differs from Figure 1 in two respects. First, retro-reflectors are used as the interferometer mirrors, rather than flat reflectors. Second, both reflectors are stationary, and scanning is accomplished by the displacement of a wedge-shaped refrac¬ tive element across one of the interferometer arms. In Figure 2, the wedge-shaped refractive element is designated by the numeral 51. The two radiation sources are 11a and 13a, the collimating lens is 15a, and the beam¬ splitter is 17a. The two reflectors are retro-reflectors 19a and 21a, both of which are stationary. Scanning is accomplished by moving wedge-shaped element 51 across th£ radiation path to vary the optical path length in one arm of the interferometer. The advantages of refractive scanning have been dis¬ cussed in my earlier application. Combining this type of 5 scanning with dual beam operation presents some unique de¬ sign considerations.- In particular, a review of the vari¬ ous earlier dual beam systems reveals that most of them cannot be used with retro-reflectors, since they are so designed as to require input and output beams which travel 1.0 at different angles within the interferometer. ' A retro- reflector will not accommodate such an arrangement because it automatically returns the beam on a path parallel to it direction of incidence. Of the previous dual beam systems with which I am fa- 15 miliar, only that of Burroughs and Chamberlain is designat ed for use with retro-reflectors. ' However, this system ha the disadvantage of requiring the input and output beams t , be transversely displaced from each other and, in fact, to -be non-overlapping at the beamsplitter. This necessitates 20 the use of a beamsplitter and retro-reflec-tors having twic the width of the usable aperture. It can thus substantial ly increase the criticality, and hence cost, of the optica components. In our case, it would also require an increas in the size of the moving refractive wedge. 5 In the design shown in Figure 2, the two beams are differentiated by angle of propagation. Since no attempt is made to detect the portions of the beams emerging on the input side of the beamsplitter, the input and output beams can be overlapping, and a significant reduction in required 0 component size will result. As shown, the beams are com¬ pletely overlapping at the input collimating element. As they travel through the interferometer, they partially separate due to their different propagation angles. An optimum arrangement would require maximum beam 5 overlap at some point within the interferometer, such as at the beamsplitter. This can be accomplished by placing an area-defining aperture at the beamsplitter. The beam displacement then is equally balanced between the input and output arms, and the overall size requirements are reduced. Such an area-defining aperture is illustrated in Fig- ure 3. The beamsplitter 17b has an aperture area 53, de¬ fined by radiation-blocking- areas 55 and 57 located along the upper and lower ends of the beamsplitter. The aperture 53 restricts the allowed combination of propagation angles and lateral positions, so as to result in the symmetrical configuration of limiting rays illustrated in the figure. This balances the displacement between the input and output optics, and thus provides the maximum overlap of the two beams within the interferometer. A possible objection to the use of angular differenti- ation for dual beam operation arises from the fact that angular field-of-view of a Fourier spectrometer has inher¬ ent limitations. If the available field-of-view must be . shared between two beams, the throughput (or total light "transmitting ability) of each beam will be reduced. This objection is substantially reduced by the embodiment of Figure 2, since the use of refractive scanning can be shown to result in an increase in the angular field-of-view approximately in proportion to the index of refraction of the refractive element. This increase is sufficient that, • even with angular beam dif erentiation, the available throughput will exceed that which can be effectively uti¬ lized by many infrared detectors, i.e., the throughput will be detector limited. We have seen that the embodiment of Figure 2 takes advantage of the large field-of-view of refractive scanning while minimizing the sizes of the various optical compo¬ nents by maximizing the overlap of the sample and reference beams. It also shares the advantage of the embodiment in Figure 1, resulting from the fact that the two beams emerge from the same side of the beamsplitter and hence are af¬ fected in the same way by its optical characteristics. The following claims are intended not only to co the specific embodiments disclosed, but also to cover inventive concepts explained herein with the maximum breadth and comprehensiveness permitted by the prior a "

Claims: What I claim is : 1. In a Fourier transform spectrometer which includes a Michelson-type interferometer having a beamsplitter which divides source radiation between a fixed-length path and a variable-length path, the combination of: radiation-propagating means for causing dual radiation beams to enter the interferometer on one side of beamsplit¬ ter; and radiation-detecting means for separately detecting the dual radiation beams as they emerge from the other side of the beamsplitter. 2. The spectrometer of Claim 1 which also includes: means for electrically subtracting the outputs of the separately detected dual radiation beams. 3. The spectrometer of Claim 1 wherein the dual beams - from the radiation-propagating means pass through the in¬ terferometer at slightly different angles. 4. The spectrometer of Claim 3 which also includes: . means for causing maximum overlap of the dual radia- tion beams at some point within the interferometer. 5. The spectrometer of Claim 4 wherein maximum over¬ lap of the dual radiation beams is caused by an area-limit¬ ing aperture at the beamsplitter. 6. That method of analyzing a material sample in .a spectrometer which includes- an interferometer beamsplitter, which method comprises: propagating a sampling radiation through the interfer¬ ometer and through the material sample, said radiation originating on one side of the beamsplitter; propagating a reference radiation through the inter- ferometer, which radiation does not pass through the mate¬ rial sample, said radiation originating on the same side o the beamsplitter as the sampling radiation; detecting the sampling radiation as it emerges from the other side of the beamsplitter; detecting the reference radiation as -it also emerges from said other side of the beamsplitter; and comparing the detected sampling and reference radia¬ tions to determine the radiation variation caused by the presence of the material sample. 7. A dual beam spectrometer comprising: a beamsplitter for dividing radiation into two optical paths; radiation-propagating means which directs two radia- tion beams along non-coinciding paths toward one side of the beamsplitter; and radiation-detecting means which detects separately the - - two radiation beams emerging from the other side of the beamsplitter. 8. The dual beam spectrometer of Claim 7 wherein one radiation beam passes through a material sample area, and the other radiation beam passes through a reference area. 9. The dual beam spectrometer of Claim 8 wherein the radiation propagating means comprises two radiation sources. 10. The dual beam spectrometer of Claim 8 wherein the two radiation beams are derived from adjacent areas of the same radiation source. 11. The dual beam spectrometer of Claim 8 wherein the two radiation beams are directed, respectively, to two optical detectors. 12. The dual beam spectrometer of Claim 11 which also comprises: means for electronically subtracting the outputs of the two optical detectors. 13. The dual beam spectrometer of Claim 8 wherein the two radiation beams propagate through the beamsplitter at different angles. 14. The dual beam spectrometer of Claims 7, 8, 12 or 13 which also comprises: retro-reflectors located at the end ' of each optical - path, taken-,by-the..radiation- after it leaves the beamsplit¬ ter, said retro-reflectors returning said radiation toward the beamsplitter; and means for varying the length of one of such optical paths with respect to the other. 15. The dual beam spectrometer of Claim 14 wherein the means for varying the length of one optical path com¬ prises a moving refractive element. 16. A dual beam spectrometer, of the type including an interferometer, comprising: means for propagating a sampling radiation beam and a reference radiation beam at different angles; beamsplitting means for dividing both radiation beams into reflected and transmitted beams directed along two interferometer arms; reflectors defining the end of each interferometer .arm and reflecting the beams back toward the beamsplitter, the length of one interferometer arm being variable for scan¬ ning purposes; two detectors located optically on the side of the beamsplitter opposite the radiation-propagating means; means for separating the sample and reference beams o said opposite side of the beamspli-tter and directing said beams, respectively, toward the two detectors; and means for electronically subtracting the outputs of the two detectors to reduce the-peak detected signal. 17. The dual beam spectrometer of Claim 16 wherein the reflectors are retro-reflectors, and wherein there is included a wedge-shaped refractive element for causing path-length scanning within the interferometer.

Inventors: DOYLE W
Applicants: DOYLE W
Application year: 1978
Main classification: G01J3
IPC codes: G01J3