INTERFEROMETRE AND FOURIER TRANSFORM SPECTROMETER

Abstract: An interferometer comprises a beam splitter (BS) and two scanning mirrors (M1, M2), which are parallel and disposed on a common slide member that can be linearly displaced. The interferometer further comprises two compensating mirrors (M3, M4), which are disposed between the beam splitter (BS) and the scanning mirrors (M1, M2). The beam splitter (BS) and each of the compensating mirrors (M3, M4) are orthogonal. The interferometer has good tolerance for displacement inaccuracies of the scanning mirrors (M1, M2). The interferometer may be used for producing a compact and inexpensive Fourier transform spectrometer.

Description: INTERFEROMETER AND FOURIER TRANSFORM SPECTROMETER Field of the Invention This invention relates to an interferometer compris¬ ing a beam splitter, which is adapted to produce a first and a second beam from an incident beam, and a first and a second reflector, which are adapted to receive, respec¬ tively, the first and the second beam and to reflect them back to the beam splitter. The invention further concerns a Fourier transform spectrometer. Background of the Invention The conventional Michelson interferometer, which essentially consists of a beam splitter and a first and a second mirror, is an example of an interferometer of the above type. In the Michelson interferometer, the beam splitter reflects part of the incident beam towards the first mirror, and transmits part of the incident beam towards the second mirror. The two mirrors reflect the beams back to the beam splitter, which combines them to a single beam which then impinges upon a detector. Depending on the difference in distance between the beam splitter and the respective mirrors, a constructive or destructive interference arises between the beams. This interference can be registered with the aid of the detec¬ tor. A Michelson-type interferometer may, for instance, be utilised in a Fourier transform spectrometer for determining the spectrum of the light from a broadband light source. In the conventional Fourier transform spec¬ trometer, the one mirror serves as scanning mirror and is linearly displaced so that the distance to the beam splitter, and hence the interference pattern en the detector, is continuously changed. In this way, an inter- ferogra can be registered by the detector. Kith the aid of Fourier transformation, the spectrum of the incident light can then be determined. An inconvenience of conventional interferometers is that the optical components have to be accurately set in relation to each other if one is to obtain an interfer¬ ence pattern fit for use. In the Michelson interferometer, the bear, splitter is so arranged that the angle of incidence of the inci¬ dent beam is 45°. Furthermore, the first and the second mirror are orthogonal, and the first and the second beam impinge upon the mirrors in the normal direction. If, say, the one mirror is so displaced that the beam does not arrive in the normal direction, the fringe density of the interference pattern will increase. When there is a considerable displacement, the fringes cannot be observed as a result of too high a density. In the Fourier transform spectrometer, the accurate setting has to be maintained during the whole linear dis¬ placement of the scanning mirror, which makes the mounting critical. In order to lower the high requirements for an accu¬ rate setting during the displacement, the mirrors may in known manner be replaced with retroreflectors. However, retroreflectors are expensive, and it is furthermore im- possible to compensate for any angle errors that may arise in production or as a result of temperature changes, ageing and the like. In order to maintain an accurate setting of the optical components, it is further known to utilise fixed mirrors for reflecting the two beams back to the beam splitter and to alter the path length of the one beam with the aid of a rotatable element, for instance a rotatable etalon. EP 0 491 435 discloses an interferome¬ ter having two parallel, opposed mirrors, which are rotated in order to alter the path length to the one fixed mirror. However, this technique has the disadvan¬ tage of the beams moving across the surfaces εf the fixed mirrors, such that extremely high requirements are placed on the quality thereof. Furthermore, the path length can only be altered to a minor extent. In the literature, there are examples of further developments of the above principles intended to enable an increase of the difference in path length between the two beams. In EP 0 314 103 and US 5,066,990, for in¬ stance, the rotating element consists of a double pendu¬ lum having a retroflector on each arm. By swinging the double pendulum, the path length of the one beam is re¬ duced, whereas the path length of the other beam is in¬ creased. However, the inconvenience associated with the retroreflectors remains. In US 5,159,405 and US 5,150,172, the rotating ele- ment consists of two parallel mirrors, between which the two beams are reflected a number of times. When the two mirrors are turned, the path length of the one beam is lengthened, whereas the path length of the other beam is shortened. However, the inconvenience of the beams moving across.the mirrors still remains. Summary of the Invention One object of the invention is to provide an inter¬ ferometer which has good tolerance for displacement inac¬ curacies of the scanning optical components, which may be of compact design, and in which the beams do not move across the optical components when the difference in path length between the beams is altered. Another object of the invention is to provide a Fourier transform spectrometer which may be of compact design. These objects are attained by an interferometer hav¬ ing the distinctive features recited in appended claim 1, as well as a Fourier transform spectrometer having the distinctive features recited in appended clairr. 8. Thus, the interferometer according to the invention is based on the principle of there being a linear dis¬ placement of the reflectors reflecting the first and the second beam back to the beam splitter in order to alter the difference in path length. In this manner, the two beams will, during the entire scanning operation, impinge upon the reflectors at the same points. Due to the si ul- taneous linear displacement of the two reflectors, the ratio between, on the one hand, the alteration of the difference in path length and, on the other hand, the displacement of the respective mirrors is 4:1, which means that a considerable alteration of the difference in path length can be achieved while retaining a compact design of the interferometer. Furthermore, the interfero¬ meter comprises a third and a fourth reflector disposed in the beam paths of, respectively, the first and the second beam between the beam splitter and, respectively, the first and the second reflector. The third and the fourth reflector as well as the beam splitter are so positioned as to be orthogonal. More specifically, the normal directions of the beam splitter, the third and the fourth reflector are orthogonal. Thus, the interferometer has a .three-dimensional configuration. The third and the fourth reflector compensate for any inaccuracies of the linear displacement of the first and the second reflec¬ tor, the third reflector compensating for the inaccura¬ cies in one direction, while the fourth reflector compen- sates for the inaccuracies in another direction perpendi¬ cular to the first. Such compensation is possible due to the fact that the first and the second reflector are fixedly interconnected, so that imperfections of the linear displacement have the same effect on the two reflectors. Finally, it should be pointed out that, in the interferometer according to the invention, the beams are reflected but a few times and the beam paths are short, which is an advantage. A few embodiments of the invention will new be described in more detail with reference to the accompa¬ nying drawings. Brief Description of the Drawings Figs 1, 2 and 3 are schematic projections in three different orthogonal directions of a first embodiment of the interferometer according to the invention, Fig. 4 is a schematic projection of a second embodi¬ ment of the interferometer according to the invention, and Fig. 5 is a schematic projection of a third embodi¬ ment of the interferometer according to the invention. Description of Preferred Embodiments Figs 1, 2 and 3 are three orthogonal projections of a first embodiment of the interferometer according to the invention. Thus, the interferometer comprises a beam splitter BS, as well as a first, a second, a third and a fourth planar mirror Ml, M2, M3 and M4, respectively. The first mirror Ml and the second mirror M2 are parallel and opposed. These mirrors constitute the scanning mirrors of the interferometer and are, to this end, disposed on a slide member (not shown) which is mounted in a ball bear- ing and which can be linearly displaced. As appears from the following, a certain deviation from a perfectly linear displacement is acceptable. The third mirror M3 and the fourth mirror M4 constitute the compensating mir¬ rors of the interferometer. The third mirror M3 is dis- posed in the beam path between the beam splitter BS and the first mirror Ml, and the fourth mirror M4 is disposed in the beam path between the beam splitter BS and the second mirror M2 in similar fashion. The third mirror M3, the fourth mirror M4 and the beam splitter BS are ortho- gonal. It should be observed that the mirrors in all the figures have been provided with arrows to facilitate a comparison between the mirror images and to indicate irregularities in the linear movement. The interferometer operates as follows. A beam Bin from a light source (not shown) impinges upon the beam splitter BS, which in known fashion reflects part of the incident beam and transmits part thereof. Thus, the beam splitter produces a first beam Bl and a second beam B2, from the incident beam. The first beam impinges upon the third mirror M3 and is reflected thereby to the first mirror Ml and hence back to the beam splitter 3S via the mirror M3. The second beam B2 impinges upon the fourth mirror M4 and is reflected thereby to the second mirror M2 and hence back to the beam splitter BS via the fourth mirror M4. The second beam B2 and the first beam Bl are, by the beam splitter BS, combined to an outgoing beam But. In Figs 1 and 3, it is indicated by dashed lines that the beam splitter BS superimposes the image of the first mirror Ml on the image of the second mirror M2 mirrored in the fourth mirror M4. Thus, Ml'33 designates the image of the first mirror Ml mirrored in the beam splitter BS, and M2'^4 designates the image of the second mirror mirrored in the fourth mirror M4. Should the slide member, on which the first and the second, mirror Ml and M2 are mounted, be slightly inaccu¬ rately set or should its position be altered in the linear displacement, such that the first and the second beam Bl and B2 do not impinge upon these mirrors in the normal direction, the fourth mirror M4 will compensate for any displacements in the Y-Z plane and the third mirror M3 will compensate for any displacements in the X-Y plane. Suppose the slide member is so displaced that the first mirror Ml is displaced in the Z-Y plane in the direction indicated by the arrow in Fig. 1, then the second mirror M2 will also be displaced in the Z-Y plane in the direction indicated by the arrow therein. The mir¬ ror images Ml ' Q and M2'^4 will then both be displaced in the direction indicated by the arrow in the mirror images, such that they will still coincide. Thus, the two beams Bl and B2 will coincide when combined with each ether by the beam splitter BS, despite the angle error. Should the first and the second mirror Ml and M2 be displaced in the X-Y plane, the third mirror compensates for the displacement in similar fashion, such that the mirror images M1'M3 and M2'33 coincide and that the first and the second beam Bl and B2 are parallel when leaving the beam splitter BS. There are a large number of configurations meeting the requirements that the beam splitter BS, the third mirror M3 and the fourth mirror M4 be orthogonal and that the first mirror Ml and the second mirror M2 be parallel and opposed. A few Examples are given below. Example 1 In the following, ή indicates the normal direction of the respective components. ήBS = (0, 1, 0) fiMl = (1, 1, -l) -β ήM2 = i (-1, -1, l; 3 ήM3 = (-1, 0, 0) ήM4 = (0, 0, -1) The incident beam Bin arrives along (1, 1, 1) . The outgoing beam But leaves along - 7 = (1, -1, 1) . The angle of incidence to the beam splitter BS and to the first and the third mirror Ml, M3 is approximately 55°, and the angle of incidence to the second and the fourth mirror M2, M4 is 90°. The beams are parallel to the space diagenals of a cube. Example 2 ήBS = (0, 1, 0) ήMl = (1, 2, -1) ήM3 * (-1, 0, 0) ήM4 = (0, 0, -ι : The incident beam Bin arrives along —?= (1, -2, 1) . The outgoing beam But leaves along — j = (-1, -2, -1) The angle of incidence to the beam splitter BS is approximately 35°; the angle of incidence to the first and the third mirror Ml, M3 is approximately 66°; and the angle of incidence to the second and the fourth mirror M2, M4 is 90°. Fig. 4 illustrates a second embodiment of the inter¬ ferometer according to the invention. This embodiment has retained the beam splitter BS and the four mirrors M1-M4 from the first embodiment. However, the second embodiment further comprises a fifth and a sixth mirror M5 and M6, which are parallel to the beam splitter BS and disposed in the beam path of the first beam Bl between the beam splitter BS and the third mirror M3 and in the beam path of the second beam B2 between the beam splitter BS and the fourth mirror M4, respectively. In this embodiment, the scanning mirrors Ml and M2 are slightly closer to one another than in the first em¬ bodiment, enabling a more compact design of the inter¬ ferometer. However, this embodiment requires two more mirrors than the first embodiment. Fig. 5 illustrates a third embodiment, which com¬ prises the same components as the second embodiment but in which the fifth and the sixth mirror are disposed between the first and the third mirror Ml, M3 and between the second and the fourth mirror M2, M4, respectively. The second and the third embodiment operate in the same way as the first embodiment. The interferometer according to the invention is ad¬ vantageously utilised in a Fourier transform spectrome- ter, which thus may be of compact and inexpensive design. It goes without saying that the above embodiments of the invention are but examples and that the invention may be modified within the scope of the appended claims. For instance, the planar scanning mirrors Ml, M2 may be replaced with other reflectors, such as retroreflectors.

Claims: CLAIMS 1. An interferometer comprising a beam splitter, which is adapted to produce a first and a second beam (Bl, B2) from an incident beam (Bin), and a first and a second reflector (Ml, M2), which are adapted to receive, respectively, the first and the second beam and to re¬ flect them back to the beam splitter, characterised in that the first and the second reflector (Ml, M2) are fixedly interconnected and adapted to be essentially linearly displaced in order to alter the difference in path length between the first and the second beam, and that the interferometer further comprises a third re- flector (M3) , which is disposed in the beam path of the first beam (Bl) between the beam splitter (BS) and the first reflector (Ml), and a fourth reflector (M4), which is disposed in the beam path of the second beam (B2) between the beam splitter (BS) and the second reflector (M2), .the beam splitter (BS) , the third reflector (M3) and the fourth reflector (M4) being orthogonal. 2. An interferometer as set forth in claim 1, wherein a fifth and a sixth reflector (M5, M6) are dis¬ posed in the beam paths of the first beam (Bl) and the second beam (B2), respectively. 3. An interferometer as set forth in claim 2, wherein the fifth and the sixth reflector (M5, M6) are disposed between the beam splitter (BS) and, respec¬ tively, the third and the fourth reflector (M3, M4). 4. An interferometer as set forth in claim 2, wherein the fifth and the sixth reflector (M5, M6) are disposed between the first and the third reflector (Ml, M3) , and between the second and the fourth reflector (M2, M4), respectively. 5. An interferometer as set forth in any one of the preceding claims, wherein the third and the fourth re¬ flector (M3, M4) are planar mirrors. 6. An interferometer as set forth in any one of the preceding claims, wherein the first and the second re¬ flector (Ml, M2) are planar mirrors. 7. An interferometer as set forth in any one of claims 2-6, wherein the fifth and the sixth reflector (M5, M6) are planar mirrors. 8. A Fourier transform spectrometer comprising an interferometer as set forth in any one of claims 1-7.

Inventors: LARSSON KAJ, LARSSON, KAJ
Applicants: LARSSON KAJ, OPSIS AB, LARSSON, KAJ
Application year: 1995
Main classification: G01J3
IPC codes: G01B9, G01J3