WO1985003122A1

WO1985003122A1 - Interferometric diode array spectrometer

Info

Publication number: WO1985003122A1
Application number: PCT/US1984/002134
Authority: WO
Inventors: Ronald R. Williams
Original assignee: Ohio University
Priority date: 1984-01-16
Filing date: 1984-12-27
Publication date: 1985-07-18
Also published as: JPS61500929A; EP0167601A1

Abstract

An interferometric spectrometer utilizing a multiple array detector (22). When the spectrometer utilizes visible light a photodiode array detector (22) is utilized and a fixed tilted mirror (17) and vertical mirror (16) in combinations with a beamsplitter (15) are utilized to produce a Michaelson interferometer with no moving parts.

Description

INTERFEROMETRIC DIODE ARRAY SPECTROMETER

BACKGROUND OF THE INVENTION ^■ Interferometers take a variety of forms. They all depend upon the production of an interference pattern which is detected by a single element detector. In the case of a Michaelson interferometer, which can be used to measure visible spectra, ultra-violet, and infrared, the interference pattern is produced by varying the dis¬ tance that a portion of a split beam of light travels. In this type of interferometer, two plane mirrors are set perpendicular to each other with a beamsplitter positioned at 45 degrees between them. The beamsplitter is fabricated so that one-half of the light striking it is transmitted and one-half is reflected. Light to be analyzed enters the interferometer perpendicular to one of the mirrors. The beamsplitter transmits 50 percent of this light to one mirror and reflects 50 percent to the other mirror. The two light beams are reflected back to the beamsplitter by the mirrors, where the beams are recombined, and then exit perpendicular to their axis of entrance. Interference is produced by modifying the path lengths travelled by one of the two beams by moving one of the mirrors toward or away from the beamsplitter. The detector monitors the exiting radiation from such an interference pattern and the signal is recorded as a function of distance travelled by the moving mirror.

There are three basic disadvantages associated with the normal Michaelson interferometer and all of these relate to the moving mirror. These disadvantages are;

1. For electronic reasons, the mirror must be moved very smoothly. Thus some type of feedback control

5 is usually necessary to minimize velocity fluctuations.

2. The moving mirror must be kept exactly perpendicular to the fixed mirror throughout the scan. Usually special air bearings are required to accomplish this,

3. The inertia of the mirror limits the speed

10 with which the mirror can travel and, consequently, limits the scan time for the spectra being analyzed.

SUMMARY OF THE INVENTION Applicant's invention combines any one of a variety of means of creating a static interference pattern 15 with an array detector which can instantaneously measure an interference pattern over an area of two dimensions.

Furthermore, in its application to a Michaelson interferometer, the use of a moving mirror is eliminated and an interference- pattern is created by a dual mirror 20 assembly with no moving parts.

While this invention will be described in detail with respect to a Michaelson interferometer, it must be kept in mind that the inventive concept is broad enough to include other means to generate an interference pattern 25 such as a Young's double slit apparatus, a Lloyd's mirror, a Fresnel's biprism, or a Billet's split lens, or other mechanisms that will accomplish the same objective.

It is therefore an objective of this invention to provide an interferometer utilizing an array detector. 30_.._. Another object of this invention is to provide such an interferometer which may be used with a variety of devices to create an interference pattern.

It is a still further object of this invention to provide such an interferometer which utilizes a 35 modified Michaelson-type interference pattern generator having no moving parts.

This, together with other objects and advantages of the invention, should become apparent in the details of construction and operation as more fully described here¬ inafter and claimed, reference being had to the accompany¬ ing drawings forming a part hereof wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWING The figure shows the arrangement of applicant's invention when used in a Michaelson-type interferometer. DETAILED DESCRIPTION OF THE INVENTION Referring now more particularly to the drawing, a source of radiation 10 is provided. Obviously, the radiation emanating from this source must be a type which is reflectable by mirrors so it would include the visible spectra, ultraviolet, and infrared. For purposes of illustration, three different rays of radiation from this source labeled 11, 12 and 13 are shown. These rays are passed through collimating lens 14 and directed to beam¬ splitter 15 where half of each ray of radiation is permitted to pass on to fixed mirror 16 and the other half of the radiation is reflected by the beamsplitter 15 to tilted mirror 17. Rays 18, 19, and 20 are the reflected one- half of rays 11, 12, and 13, respectively. Tilted mirror 17 is shown tilted but perpendicular to the same plane of the drawing as is the fixed mirror 16. It must be recognized that tilted mirror 17 may also be additionally tilted so that it is not perpendicular to the plane of the drawing and thus will produce a two-dimensional inter¬ ference pattern which may be read on a two-dimensional photodiode array detector. The photodiode array detector shown in this drawing is essentially in the single dimension of the plane of the drawing.

Since ray 20 must travel a greater distance than ray 13 when ray 20 is reflected back from tilted mirror 17, and reengages'the reflected ray 13 from the fixed mirror at the beam splitter 15, it will have travelled a greater distance than will have reflected ray 13 and thus will be recombined out of phase. (The tilt of mirror 17 is exaggerated for purposes of illustration and actually rays 18, 19, and 20, though angularly displaced, when they return to the beamsplitter the amount of displace¬ ment is much smaller than the beam divergence and therefore the interference pattern is not affected. For illustration purposes the lines are shown to return on the same path that they came from the beamsplitter.) Therefore, recombi¬ nation of beams 13 and 20 will produce a signal of inter¬ mediate intensity when the recombined rays 13 and 20 travel to the photodiode array detector. Rays 12 and 19, having travelled exactly the same distance from the reflective mirror back to the beamsplitter 15, will be exactly in phase and the recombined rays 12 and 19 will produce a signal of large intensity at the photodiode array detector. Likewise, ray 18 will have travelled a shorter distance than the other half of its ray 11 and when recombined rays 11 and 18 will create a signal of some intermediate intensity since these two rays will be out of phase. The thus recombined rays are transmitted through a focuss- ing lens 21 and displayed on the photodiode array detector

22 with recombined rays 13 and 20, 19 and 12, and 18 and 11 being received at various positions on photodiode array detector 22. ^' While these rays have been described, it is obvious that an infinite number of combined rays will strike the photodiode array detector 22.

In operation, a reading from the photodiode array detector 22 is taken with no sample at focal point

23 and then the sample is inserted at focal point 23 and a reading is taken. The ratio between the readings with- out the sample and the reading with the sample at focal point 23 provides the transmittance spectrum. In use, photodiode array detectors having as many as 4,000 or more different detectors are used so that a great amount of different information can be obtained rapidly and much more inexpensively than would be obtained if the normal mirror of a Miσhaelson inferometer were moved backwards and forwards. If tilted mirror 17 is not positioned at right angles to the plane of the drawing and a two-dimensional photodiode array detector having a large area is employed, even more information may be obtained. Two-dimensional detectors are currently commercially available having 256² elements.

Since there is no moving mirror, all of the problems associated with the normal Michaelson interfero¬ meter are obviated. All of the electronic and mchanical apparati needed for smooth, perpendicular and reproduc- ible motion are not required. More importantly from an experimental point of view, the time necessary to acquire complete spectra is determined by the detector rather than the spectrometer and thus the complete spectra are available instantaneously. Since the Michaelson inter- ferometer version of applicant's invention has no moving parts, it is virtually immune to vibrational problems and is suitable for application in harsh environments.

Reproducibility is insured because of the fixed detector spacing of the photodiode array. In other words, the array always samples the interferogram in exactly the same places. Finally, there are no dimensional effects on the signal caused by electronic filtering as is common in- normal interferometers.

While this invention has been described in detail in connection with a Michaelson-type interfero¬ meter utilizing radiation in the visible spectrum, ultraviolet, and infrared, it should be understood that the interference pattern generator described above may be replaced by a Young's double slit apparatus, a Lloyd's mirror, a Fresnel's biprism, and a Billet's split lens, all of which are described in The Theory of Light, by Thomas Preston, and published by MacMillan & Company, London, 1924, and all of which can generate interference patterns by suitable detection. Furthermore, the spectral range studied by the spectrometer can easily be altered by changing detector arrays provided a suitable interference pattern can be

-6- generated. For example, diode arrays, sensitive to x-rays, are readily- available and an x-ray spectrometer of this invention would utilize Young's double slit apparatus with these x-ray detectors.

While this invention has been described in its preferred embodiment, it is appreciated that variations thereon may be made without departing from the proper scope and spirit of the invention.

Claims

1. An interferometric spectrometer comprising a source of radiation, means for forming a static interference pattern from said radiation, a multiple array detector on which said interference pattern may be received.

2. The interferometric spectrometer of claim 1, wherein the means for forming an interference pattern is Young's double slit apparatus.

3. The interferometric spectrometer of claim 1, where¬ in the means for forming an interference pattern is Lloyd's mirror apparatus.

4. The interferometric spectrometer of claim 1, where¬ in the means for forming an interference pattern is Fresnel's bipris .

5. The interferometric spectrometer of claim 1, wherein the means for forming an interference pattern is Billet's split lens.

6. An interferometric spectrometer comprising a source of electromagnetic radiation which is capable of being reflected from a mirror, a collimating lens through which said radiation may be passed, a beamsplitter through which said radiation may be split into a first portion and into a second portion, a fixed mirror positioned at right angles to the original path of said radiation and adapted to reflect said first portion of said radiation back to said beamsplitter, a tilted mirror not at right angles to the path of said radiation and adapted to reflect said second portion of said radiation back to said beamsplitter whereby the combined reflected radiation forms an interference pattern, a focussing lens through which said interference pattern of said radiation may be passed, a multiple photodiode array detector on which said interference pattern may be received.

7. The interferometric spectrometer of claim 6, wherein .

-8- said source of electromagnetic radiation produces radiation including the visible, ultraviolet, and infrared spectra.

8. The interferometric spectrometer of claim 6, where¬ in said fixed mirror and said tilted mirror are positioned at right angles to the same plane.

9. The inter^'ferometric spectrometer of claim 6, wherein said tilted mirror is not positioned at right angles to the same plane as said fixed mirror.

. . claim 1 amended, other claims unchanged (1 page )]

1. An interferometric spectrometer comprising a source of radiation, means for forming a static interference pattern from said radiation, means for introducing and removing a sample to be^" examined in the path of said radiation, a multiple array detector on which said interference pattern may be received.

4. The interferometric spectrometer of claim 1, where¬ in the means for forming an interference pattern is Fresnel's biprism.

7. The interferometric spectrometer of claim 6, wherein