GB2106665A

GB2106665A - Spectroscopic analysis

Info

Publication number: GB2106665A
Application number: GB08227191A
Authority: GB
Inventors: Noel William Frank Stephens
Original assignee: SPECTRON DEVELOPMENTS Ltd
Current assignee: SPECTRON DEVELOPMENTS Ltd
Priority date: 1981-09-23
Filing date: 1982-09-23
Publication date: 1983-04-13

Abstract

A birefringent variable-retardation device comprises a fixed inclined plate 40 of positive birefringence material, a fixed plate 42 of negative birefringence inclined oppositely, and a pair of plates 44, 45 also inclined in opposed directions and of positive and negative birefringence material respectively. The plates 44, 45 are rotatable about an axis 46 to vary the retardation imposed by the device upon a beam 4 of radiation. The device may be used in a birefringent Fourier transform spectrometer where it may be replaced by a device having one fixed, inclined birefringent plate and one rotatable plate (28 in Fig. 3 - not shown). The rotating plate devices may also be replaced by a liquid crystal device (50 in Fig. 5 - not shown). <IMAGE>

Description

SPECIFICATION Spectroscopic analysis DESCRIPTION The invention relates to Fourier transform spectroscopy and to birefringent retardation devices which can be used in Fourier transform spectroscopy.

Birefringent optical elements known as wave plates comprise plates, usually of such crystalline materials as quartz or mica. The action of such a plate can be described in terms of resolution of the polarization vectors of polarized light incident normally on one face of the plate, in directions containing mutually perpendicular fast and slow axes of the plate, which axes lie in the plane of the plate. The polarization vectors so resolved constitute the so-called oridinary and extraordinary rays within the plate and have different velocities through the plate so that the vectors of the light leaving the opposite face of the plate have experienced a relative phase shift. In a half waveplate, this phase shift or retardation amounts to one half of the wavelength of the incident light, but the plate may impose a different amount of phase shift.For example, a multiwave plate effects a phase shift through a plurality of wavelengths.

These birefringent optical elements have been used in Fourier transform spectroscopy in the form of devices known as Soleil-Babinet compensators which comprises a pair of wedged plates of birefringent material mounted for movement to vary the retardation introduced into a beam of radiation passing through the plates. By moving the plates in a systematic manner, and directing the beam of radiation onto a suitable detector, an output from the detector is obtained and can be subjected to a Fourier transform procedure to obtain the spectral content of the radiation beam.

However, Fourier transform spectroscopic apparatus including a Soleil-Babinet compensator is structurally less complicated than interferometric apparatus but has the disadvantages that the birefringent plates must be subjected to translational movement, which is mechanically difficult to provide for, and that the angular field of light collection of the apparatus is restricted.

According to the present invention there is provided an apparatus, for Fourier transform spectroscopy, comprising means for producing a beam of polarised radiation to be examined, a birefringent variable-retardation device positioned to transmit the radiation beam, a polarising analyser for the beam transmitted by the retardation device, and a detector for receiving the beam from the analyser to produce an output upon which Fourier transform can be performed, the retardation device comprising at least one birefringent plate and the angle of incidence of the beam upon the plate being selectively variable to vary the retardation imposed upon the beam by the retardation device; and a method of Fourier spectroscopy, the method having the steps of passing a beam of polarised radiation through a birefringent variable-retardation device comprising at least one birefringent plate, varying the angle of incidence of the beam upon the plate, passing the beam transmitted by the device through a polarising analyser to a detector, and performing a Fourier transform on the detector output.

The invention also provides a birefringent variable-retardation device which is particularly, but not exclusively, suitable for use in the above apparatus as an alternative to the device described above. The birefringent variable-retardation device according to the present invention comprises a plurality of birefringent plates positioned for transmission of a beam of radiation through the plates in succession, each plate being oriented to the direction of propagation of the beam so as to impose a retardation upon the beam and the angel of incidence of the beam upon at least one plate being selectively variable to vary the retardation imposed upon the beam by the device.

The retardation device may alternatively be a liquid crystal device and the invention also provides an apparatus for Fourier transform spectroscopy, comprising means for producing a beam of polarised radiation to be examined, a birefringent variable-retardation device positioned to transmit the radiation beam, a polarising analyser for the beam transmitted by the retardation device, and a detector for receiving the beam from the analyser to produce an output upon which a Fourier transform can be performed, the retardation device comprising a layer of liquid crystal material and having means for applying a selectively variable electrical potential to the liquid crystal material to vary the retardation imposed upon the beam by the retardation device, and a method of Fourier transform spectroscopy, the method having the steps of passing a beam of polarised radiation to be examined through a retardation device comprising a layer of liquid crystal material, applying an electrical potential to the material, varying the electrical potential to vary the retardation imposed upon the beam by the device, passing the beam transmitted by the device through a polarising analyser to a detector, and performing a Fourier transform on the detector output.

Advantageously, the plates are mounted with their fast or their slow axes perpendicular to the direction of propagation of the beam and at least one plate is preferably rotatable about its fast or its slow axis to vary selectively the angle of incidence of the beam upon the plate.

Specifically, for rotation of a tilting plate device about its optic axis, the retardation N, representing a number of wavelengths of the radiation, can be expressed by:

where: B is the tilt angle, d is the physical thickness of the plate, n0 is the refractive index of the extraordinary ray, nO is the refractive index of the ordinary ray, and A is the wavelength of the light or other radiation.

The fast or slow axis will coincide with the optic axis for uniaxial crystals depending upon whether the material has negative or positive birefringence.

The acceptance angle of light incident on the plate decreases as the tilt angle increases so the usefulness of a single tilting plate is limited and advantageously the birefringent device comprises two birefringent plates located to intercept a radiation beam, the fast axis of one plate and the slow axis of the other plate being parallel to one another and perpendicular to the direction of propagation of the radiation beam, and the slow axis of the one plate and the fast axis of the other plate being inclined to the propagation. The retardation imposed by the device can be selectively varied by pivoting the or each plate about the axis thereof perpendicular to the propagation direction, to vary the inclination of the inclined axis.

The acceptance angle and range of the device can be improved, as can linearity of retardation with tilt, by the addition of two further inclined birefringent plates, two of the four plates then employed having negative birefringence and the other two having positive birefringence, and the optic axes of the plates being parallel. One of each of the positive and negative plates can be fixed and the other two rotate or tilt together. The fixed and movable pairs of plates can be side by side or the movable pair may be between the two fixed plates.

The bias tilt angles between the two pairs of plates are preferably substantially the same, the angle being selected to obtain the best performance from plates of the particular material or materials being used. Typically the bias angle will be between 1 5" and 30 and will be adjustable to correct for minor variations in the optical properties of the crystals and minor errors of alignment and mounting.

The material of the birefringent plates is preferably one that has constant birefringence across the spectral range under analysis. This is of particular importance at high spectral resolutions.

Variations in birefringence over the spectral range cause undesirable distortion of the spectral transmission function, for which compensation can however be made in computing the spectral distribution of the radiation under test provided the relationship between birefringence and wavelength is known.

The retardation due to each plate will be in accordance with the equation given above but in practice the physical thicknesses d1, d2 of the negative and positive plates of the or each pair of plates will not generally be quite equal but, for maximum linear range, the following relationship, in which 8 is the bias angle of the pair of plates, will normally hold:

In any retardation device according to the invention the total number of spectral positions resolved substantially equals the difference, or total variation, in the retardation N.Thus for a spectral interval A1 to A2, the number of resolved spectral positions will equal the retardation expressed in number of waves at the wavelength given approximately by: A1A2 = A, Embodiments of the invention are described below, by way of example, with reference to the drawing, in which: Figure 1 is a schematic side view of a spectroscopic apparatus; Figure 2 is a graph showing variation of intensity with wave number of radiation treated in the apparatus of Fig. 1; and Figures 3, 4 and 5 are respectively schematic side views of three different birefringent devices which may be used in the apparatus of Fig. 1.

The illustrated apparatus comprises a source 2 of radiation, typically in the ultra-violet, visible, or infra-red wavelength ranges, the spectral content of which is to be analysed. A beam 4 of radiation from the source 2 is transmitted through device 8 which transmits the beam 4, after polarization by the polarizer 6, at two different speeds in the mutually perpendicular directions of the polarizing vectors, dividing the beam into the so-called ordinary and extraordinary beams.

The device 8, which may be of zinc sulphide or other material appropriate to the spectral range of the beam 4, has different refractive indices for the ordinary and extraordinary beams, that is, the quality of birefringence.

The beam leaving the device 8 consequently has its ordinary and extraordinary beams out of phase by an amount dependent on the frequency of the beam and the effective thickness of the device. The beam leaving the device 8a is transmitted through an analyser 10 and is then incident upon a detector 1 2 which provides an electrical output at 14 dependent on the intensity and spectral content of the incident beam.

With the polarizer 6 and analyser 10 crossed or parallel the beam incident on the detector 1 2 has a spectral transmission profile such that a substantially sinusoidal curve is obtained by plotting intensity against wave number, as indicated in Fig. 2. The curve is in fact cosinusoidal when the origin is at zero wave number. The spectral period (16 on Fig. 2) is inversely proportional to the plate thickness.

By continuously varying the effective thickness of the device 8 a Fourier cosine transform can be built up in a given spectral band. By performing the inverse transform on the electrical output at 14, the spectral content of the incident radiation can be established.

The device 8 can be constituted by a device 28 of the kind shown in Fig. 3, which comprises two birefringent plates 30, 32. Plate 30 is inclined to the direction of the incident beam 4 about its fast axis, that is, its fast axis is perpendicular to the beam direction and the slow axis inclined thereto. Plate 32 has its fast axis inclined to the beam direction by being tilted about its slow axis which is parallel to the fast axis of plate 30. Depending on whether the material of the plate has positive or negative birefringence, it is more effective to vary the angle of tilt about the fast axis of plate 30 or the slow axis of plate 32. In the device illustrated, the inclination of the plate 32 is continuously cyclically varied about its slow axis through a predetermined angular range and back again by a drive device schematically shown at 35.Both plates however may be rotatable about the axis perpendicular to the beam 4, the movement of one plate being for example adjustable in steps to change the range of retardations swept by the continuous rotation of the other. An equalizing plate 34 can be included in the device if desired.

The retardation device 8 may alternatively be constituted by the device 38 of Fig. 4 in which the beam 4 passes successively through a fixed inclined plate 40 of positive birefringence material, a fixed plate 42 of negative birefringence inclined in the opposite direction, and a pair of plates 44, 45 also inclined in opposed directions and of positive and negative birefringence material respectively, plates 44 and 45 being continuously cylically rotated together about an axis 46 by a drive mechanism 47, in the same way as plate 32 in the device of Fig. 3. The axis 46 is perpendicular to the direction of the beam 4 and parallel to the optic axes of all four of the plates. The bias angles between the two pairs of plates 40, 42 and 44, 45 are alike.The device of Fig. 4 gives a very nearly linear change in retardation with rotation of the plates 44, 45, and also a range of retardation extending over a large number of wavelengths.

An equalizing plate 48 can be provided in the device of Fig. 4. For the purpose of extending the resolution within a given spectral range, provision can be made for varying the thickness of the equalization plates 34, 48, as by selection of the plate from a range of plates of different thicknesses.

The devices 28 and 38 have been described as including plates 30, 32, 40, 42, 44, 45 of uniaxial crystalline material. The plates may also be formed by biaxial material. In this case, the line equivalent to the optic axis is a line which bisects the angle between the two optic axis of the crystal and the plates are mounted accordingly.

Fig. 5 shows a liquid crystal display device 50, known as such, which may also be employed in the apparatus of Fig. 1 instead of the device 8. The device 50 comprises a spaced pair of parallel glass plates 52, 54 having transparent electrode structures 56, 58 respectively on their inner surfaces. A nematic liquid crystal material 60 is sealed between the plates. A control voltage source 62, applies a potential difference which varies in a cyclic manner to the electrode structures 56, 58, and this causes the molecular structure of the material 60 to be varied between one imposing zero retardation, on the beam 4, to one of maximum retardation.

Claims

1. A birefringent variable-retardation device comprising a plurality of birefringent plates positioned for transmission of a beam of radiation through the plates in succession, each plate being oriented to the direction of propagation of the beam so as to impose a retardation upon the beam and the angle of incidence of the beam upon at least one plate being selectively variable to vary the retardation imposed upon the beam by the device.

2. A device according to claim 1, in which the plates are mounted with their fast or their slow axes perpendicular to the direction of propagation of the beam.

3. A device according to claim 2, in which at least one plate is rotatable about its fast or its slow axis to vary selectively the angle of incidence of the beam upon the plate.

4. A device according to claim 3, comprising two plates the fast or slow axis of one plate being parallel to the fast or slow axis of the other plate and perpendicular to the direction of beam propagation, one plate being rotatable about the axis thereof which is perpendicular to the propagation direction and the other plate being inclined at a fixed angle to the propagation direction.

5. A device according to claim 4, in which the plates are of material having the same sign of birefringence and the fast axis of one plate is perpendicular to the slow axis of the other plate.

6. A device according to claim 3, comprising first, second, third and fourth plates mounted with their fast or their slow axes perpendicular to the direction of beam propagation, the first and second plates being inclined at a fixed angle to the direction of beam propagation and the third and fourth plates being rotatable about their axes perpendicular to the direction of beam propagation.

7. A device according to claim 6, in which the third and fourth plates have a fixed orientation to each other.

8. A device according to claim 7, in which the first and third plates are of material of the same sign of birefringence and the second and fourth plates are of material of the opposite sign to the first and second plates, the optic axes of the plates being aligned and perpendicular to the direction of beam propagation.

9. A device according to any one of claims 1 to 8, including an equalising plate allowing the retardation of the device to be adjusted to zero.

10. An apparatus for Fourier transform spectroscopy, comprising means for producing a beam of polarised radiation to be examined, a birefringent variable-retardation device positioned to transmit the radiation beam, a polarising analyser for the beam transmitted by the retardation device, and a detector for receiving the beam from the analyser to produce an output upon which a Fourier transform can be performed, the retardation device comprising at least one birefringent plate and the angle of incidence of the beam upon the plate being selectively variable to vary the retardation imposed upon the beam by the retardation device.

11. An apparatus according to claim 10, in which the birefringent plate is rotatable about the fast or the slow axis of the plate, the axis extending perpendicularly to the direction of beam propagation.

1 2. An apparatus for Fourier transform spectroscopy, comprising source means for producing a beam of polarised radiation to be examined, a birefringent variable-retardation device according to any of claims 1 to 9 positioned to transmit the radiation beam, a polarising analyser for the beam transmitted by the retardation device, and a detector for receiving the beam from the analyser to produce an output upon which a Fourier transform can be performed.

13. An apparatus for Fourier transform spectroscopy, comprising means for producing a beam of polarised radiation to be examined, a birefringent variable-retardation device positioned to transmit the radiation beam, a polarising analyser for the beam transmitted by the retardation device, and a detector for receiving the beam from the analyser to produce an output upon which Fourier transform can be performed, the retardation device comprising a layer of liquid crystal material and having means for applying a selectively variable electrical potential to the liquid crystal material to vary the retardation imposed upon the beam by the retardation device.

14. A method of Fourier transform spectroscopy, the method having the steps of passing a beam of polarised radiation through a birefringent variable-retardation device comprising at least one birefringent plate, varying the angle of incidence of the beam upon the plate, passing the beam transmitted by the device through a polarising analyser to a detector, and performing a Fourier transform on the detector ouptut.

1 5. A method of Fourier transform spectroscopy, the method having the steps of passing a beam of polarised radiation to be examined through a retardation device comprising a layer of liquid crystal material, applying a layer of liquid crystal material, applying an electrical potential to the material, varying the electrical potential to vary the retardation imposed upon the beam by the device, passing the beam transmitted by the device through a polarising analyser to a detector, and performing a Fourier transform on the detection output.

1 6. A birefringent variable-retardation device substantially as hereinbefore described with reference to Fig. 3 or Fig. 4 of the drawings.

17. An apparatus for Fourier transform spectroscopy substantially as hereinbefore described with reference to Fig. 1 of the drawings taken with any one of Figs. 3, 4 and 5 of the drawings

1 8. A method of Fourier transform spectroscopy substantially as herein described with reference to Fig. 1 of the drawing taken with any one of Figs 2, 3 and 4.