POLARIZATION INTERFEROMETER

• US 3,728,030 A
• Filed: 06/22/1970
• Issued: 04/17/1973
• Est. Priority Date: 06/22/1970
• Status: Expired due to Term

• Current Assignee: Roland C. Hawes
• Original Assignee: Roland C. Hawes
• Inventors: Hawes, Roland C.
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): Clark, Conrad
• Application Number:
  US05/048,415
• Time in Patent Office: 1,030 Days
• Field of Search: 356/106-113,114-117,33
• US Class Current: 356/453
• CPC Class Codes: G01J 3/4537 Devices with refractive scan
• CPC Class Codes: G01N 21/19 Dichroism

Abstract

Fourier spectroscopy is used in the measurement of circular dichroism, the method involving the use of an interference polarization modulator which is characterized by production of negligible amplitude modulation in the absence of dichroism in the optical train that follows the modulator and in the detector. Either linear or circular dichroism in that region of the instrument will convert the polarization modulation into amplitude modulation. The polarization modulation is characterized by a different frequency for each wavelength of the radiation, thus the signals caused by the interaction of the radiation with dichroic sample may all be recorded simultaneously and may subsequently be '"'"''"'"''"'"''"'"'unscrambled'"'"''"'"''"'"''"'"' to derive the inverse Fourier transform of the ensemble of frequencies constituting the complete signal, and thus obtaining a transmission spectrum corresponding to the dichroism. The transmission spectrum in turn, in the case of the circular dichroism, may be converted into dichroism by dividing by the ordinary transmission spectrum (corresponding to ordinary absorption) which may be derived by ordinary Fourier spectroscopy.

Claims

1. In dichroism measurement apparatus, the combination comprising, a. a source of electromagnetic radiation of different wavelengths lambda , b. interferometer means for processing source radiation and wherein the phase between orthogonally polarized beams is varied to provide a resultant beam characterized, for each wavelength, by ellipticity that alternates between left and right circular polarization and between which the beam polarization becomes linear in one direction as the ellipticity alternates from left to right circular polarization, and linear in another direction as the ellipticity alternates from right to left circular polarization, the characteristic frequency Nu a of such alternation varying as a function of the wavelength, c. a sample space located for effecting passage of the elliptically polarized beam through a dichroic sample in that space, the sample differentially absorbing the alternately polarized radiation of a characteristic set of wavelengths lambda , and d. a beam intensity detector located in the path of the beam passing from the sample space and characterized as having signal output that varies in intensity with frequency Nu a when said sample is in said space, said output adapted for processing to produce dichroic spectra varying with wavelength lambda .

2. The combination of claim 1 wherein said means comprises a beam splitter located for passing and reflecting source light components in two beams each of which is reflected from relatively movable reflectors, and a linear polarizer in the beam path between the source and said splitter.

3. The combination of claim 2 including said sample in said space.

4. The combination of claim 2 including apparatus for processing said detector output to produce said dichroic spectra.

5. The combination of claim 4 wherein said apparatus includes a digitizer connectEd to receive and digitize a version of the detector output, and means operatively connected with the digitizer to derive data representing dichroic spectra as a function of wavelength lambda .

6. The combination of claim 2 including actuating means for effecting such relative movement of the reflectors to control said frequencies Nu a.

7. The combination of claim 6 wherein said actuating means is operatively connected to one of the reflectors which receives light from the beam splitter.

8. The combination of claim 7 wherein said polarizer, splitter and reflectors are located to orient the light polarization vectors in a three dimensional orthogonal system having X, Y and Z axes, and characterized in that the polarization vector of light traveling in the direction of one of the axes from the polarizer to the splitter and through the splitter toward one reflector is oriented in the direction of a second of the axes, the polarization vector of light returning to the splitter after reflection by said one reflector is oriented in the direction of a third of the axes, and the polarization vector of light reflected by the splitter in the direction of said second axis and traveling toward and away from the other reflector is oriented in the direction of the said first axis.

9. In apparatus for measuring dichroism of a sample, the combination comprising a. first means to produce an electromagnetic radiation beam passing successively through the following states with an alternation frequency Nu , in which Nu varies with the wavelength of the radiation:

i. circularly polarized in one rotary direction ii. elliptically polarized iii. linearly polarized in one linear direction iv. elliptically polarized v. circularly polarized in the opposite rotary direction vi. elliptically polarized vii. linearly polarized in another linear direction at an angle to said one direction viii. elliptically polarized b. said beam having little or no modulation of its intensity at any of the frequencies Nu , c. a sample space located for effecting passage of the beam through a dichroic sample in that space and characterized as differentially absorbing the polarized radiation of a characteristic wavelength lambda , and d. processing means including a detector located in the path of the beam passing from the sample space and characterized as having signal output that varies with frequency when said sample is in said space, said processing means processing said output to produce dichroic spectra varying with wavelength lambda .

10. The combination of claim 9 wherein said first means includes a polarization interferometer having movable beam reflecting structure.

11. The combination of claim 2 wherein said detector output defines an interferogram function F ( Nu a) and said processing means includes a computer operable to derive therefrom a Fourier transmittance dichroism spectrum Delta TL R( lambda ).

12. The combination of claim 11 wherein said computer is also operable to derive a Fourier spectroscopy transmittance spectrum T( lambda ) for ordinary absorption, and to divide Delta TL R( lambda ) by T( lambda ) to derive an approximate dichroism spectrum Delta AL R( lambda ).

13. The combination of claim 11 wherein the computer derives said spectrum by effectively performing the operation

14. The combination of claim 13 wherein said last named means includes apparatus for transmitting auxiliary radiation beams to said interferometer means for processing thereby to produce measurable fringes modulated as a function of relative movement of the reflectors.

15. The combination of Claim 4 in which the directions of polarization of the beams recombined by the interferometer means are orthogonal.

16. The combination of claim 6 wherein the direction of polarization of the light passed by the beam splitter is unchanged on being returned to the beam splitter, while the direction of polarization of the light reflected by the beam splitter is rotated by 90* on being returned to the beam splitter.

17. The combination of claim 6 wherein the direction of polarization of the light reflected by the beam splitter is unchanged on being returned to the beam splitter, while the direction of polarization of the light passed by the beam splitter is rotated by 90* on being returned to the beam splitter.

18. The combination of claim 9 wherein said processing means includes a circular dichroism display.

19. The combination of claim 9 wherein said processing means includes a linear dichroism display.

20. Apparatus for measuring dichroism by Fourier spectroscopy, comprising:

a. scanning interferometer means for receiving and molulating the polarization state of a linearly polarized beam of broadband electromagnetic radiation and operable to produce separate but coherent linearly polarized beams of approximately equal intensity, the separate beams having orthogonal polarization directions after recombination, one of said beams being progressively retarded in phase with respect to the other, the retardation rate being different for different wavelengths within the wavelength band of the radiation, said interferometer means combining said beams, with substantially no amplitude modulation of the combined beams, for subsequent passage through a sample, whereby amplitude modulation of the intensity of the combined beams results only if the sample exhibits dichroism at wavelengths within the band, b. detector means responsive to the beams passed through the sample for producing an electrical signal with frequency components corresponding to wavelengths at which the sample exhibits dichroism, and c. means responsive to said electrical signal to derive an output signal representative of an inverse Fourier transform of said electrical signal, said last named means including means for determining the relative retardation of said one beam with respect to said other beam.

21. The combination of claim 20, in which said means to determine the relative retardation comprises, a. means to produce a first auxiliary optical beam passing through the interferometer means and falling on an auxiliary detector for producing a signal identifying the zero order point of the interferometer scan, and b. means to produce a second auxiliary optical beam passing through the interferometer means and falling on an auxiliary detector for producing a signal providing an accurate measure of the difference in optical paths of said two coherent beams.

22. The combination of claim 20 including means to derive the inverse Fourier transform of the absorption interferogram produced by the sample, and means to effectively divide said output signal by said inverse Fourier transform of said absorption interferogram produced by the sample.

23. The combination of claim 20 including electrical filter means to isolate said frequency components, and output means to register the amplitude of said frequency components.

24. The combination of claim 20 wherein said interferometer means has an even number of beam reflectors in one arm thereof and an odd number of reflectors in the other arm thereof.