Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods

Definitions

• This invention relates to optical measuring instruments and, in particular, to apparatus for measuring the refractive index of gases.
• optical apparatus for measuring the refractive index of gaseous media including radiation source means to produce a beam of polychromatic radiation, beam splitter means to produce a pair of component beams from said beam of polychromatic radiation, dual chamber cell means have first and second chambers, deflector means to direct a first of said pair of component beams through said first chamber and said second of said pair of component beams through said second chamber, recombining means to recombine said first and said second component beams after passage through said chamber, dispersion means differentially to deflect radiation of different wavelengths in said first and second component beams after recombination and sensing means disposed to received radiation deflected by said dispersion means.
• Figure 1 depicts an optical measuring instrument which uses several laser wavelengths.
• Laser light from a source L is reflected by a mirror M ⁇ and is then incident on a fused silica beam splitter BS which has a semi-reflecting front surface and a fully reflecting back surface.
• the reflected beam 1 from the front surface travels down the outer arm 3 of a dual compartment cell 5 and is returned by a corner cube reflector 7 along a parallel path to the beam splitter.
• the transmitted beam 9 through the beam splitter is reflected by the fully aluminised back surface and travels down the inner arm 11 of the instrument before being returned by the cube corner reflector 7 to the beam splitter BS where it is reco bined with the beam that has travelled along the outer arm.
• This configuration of refractometer is used to determine the change in optical path length between the two arms by using the measurement of the fringe fractions at each of the wavelengths used. For example, when a dual wavelength laser is used emitting wavelengths X x and ⁇ 2 any change in optical path length (dl) in either of the arms of the instrument is given by (if air dispersion is ignored):
• x and 2 are the integer numbers of the interference orders and ⁇ x and ⁇ 2 are the fringe fractions for each wavelength.
• This sequence of fringe fractions will be repeated every time the optical path length changes by ⁇ 1 ⁇ 2 /( ⁇ 1 - ⁇ 2 ) .
• a third fringe fraction from an additional wavelength can be used or an approximate value of dl is employed which is determined by other means. Hence any change in optical path length can be obtained in this way without using continuous fringe counting.
• the inner chamber of the cell has a known constant optical path length whilst the outer chamber contains the air requiring refractive index measurement. From both the measurement of the fringe fractions under these conditions and also when the refractometer has an identical optical path length in both arms, a value for the refractive index of air [n 1 ( ⁇ >] at wavelength ⁇ can be precisely determined from the simple equation:
• n x (X) n 2 ( ⁇ ) + dl( ⁇ ) + ⁇ (2)
• n 2 ( ⁇ ) is the refractive index of the medium contained in the inner cell chamber at wavelength ⁇
• dl( ⁇ ) is the induced path length change
• ⁇ is a correction for any additional optical path length changes induced during the measurement procedure.
• the second of these alternatives is preferable since dual wavelength lasers are readily available with the same common wavelength (633nm) as that used in length measuring interferometers which, for the highest accuracy length measurements in air, would require correspondingly accurate air refractive index values.
• a suitable value of dl( ⁇ ) can be easily obtained using inexpensive and rugged pressure and temperature sensors with respective accuracies of ⁇ 500 Pa and ⁇ 1°C.
• the above embodiment of the invention uses a gas cell that has an inner chamber with a known constant optical path length and this must be maintained to within about 3nm over the ambient ranges of atmospheric pressure and air temperature in order to achieve a measurement accuracy of 1 x 10 ⁇ 8 in air refractive index.
• This can be realised by using a cell which has either a permanently evacuated central chamber which incorporates a getter pump for maintaining and monitoring the vacuum or a central chamber that is filled with dry gas at atmospheric pressure.
• Both of these approaches require an initial measurement of the length (1) of each cell to an accuracy of about ⁇ l ⁇ m (equivalent to an uncertainty of 1 x 10 ⁇ 9 in refractive index measurement using a cell of length 31cm) followed by the insertion of the cell into the refractometer and the evacuation of both cell chambers to determine the dual wavelength "zero" fringe fractions which should be identical since both optical paths are exactly equal. This ensures that there are no anomalous dispersion effects which could lead to measurement errors.
• the inner chamber is sealed with the vacuum being maintained and monitored by a getter pump and air is leaked into the outer chamber following which the dual wavelength fringe fractions are determined.
• Atmospheric pressure and air temperature are also measured and these values are used to calculate an approximate air refractive index value from Edlen's equation which, with the measurement of the cell length, allows the change in optical path length Cdl( ⁇ )] to be calculated to within a few fringes.
• the refractometer is now ready for the continuous measurement of refractive index with the requirement for weekly or monthly repeat determinations of the "zero" fringe fractions.
• the inner chamber is filled with dry air from a gas cylinder following which this chamber is sealed at atmospheric pressure.
• the dual wavelength fringe fractions together with the pressure and temperature of the air in the cell are similarly determined.
• n2( ⁇ ) n3( ⁇ ) + dl( ⁇ ) + ⁇ ( 3)
• the refractometer is now ready for the continuous measurement of air refractive indices with similar measurement requirements for the "zero" fringe fractions as those discussed previously.
• fused silica is used to fabricate the cell. This results is an insignificant refractive index correction due to variations in the cell volume induced by ambient temperature changes.
• the first type of cell has the advantage that the refractive index of the medium is exactly 1.000 and.
• a refractometer can be operated in two modes to produce refractive index data.
• the first version shown in Figure 1, allows the optical path in the inner arm to be varied by a few interferometer fringes in order to determine the fringe fractions.
• This path length variation which is illustrated in Figure 2, is produced by uniformly varying the pressure in the single compartment cell and monitoring the induced change in optical path as a function of time.
• Figure 2 shows two amplified interferometer signals produced by the photodetectors located in the output beams of the refractometer with each photodetector monitoring the interfering output from each wavelength as, in the illustration, the optical path length is varied.
• a clock which generates equally spaced pulses is reset.
• fringe fractions at each wavelength may now be easily determined as the simple fractions ⁇ N j /N j and ⁇ N 2 /N 2 for and ⁇ 2 respectively. In this way fringe fraction data is obtained from which the air refractive index can be determined using those techniques described earlier.
• the second embodiment of the invention is shown in Figure 3 which illustrates the required change in the arrangement for dual wavelength operation.
• the plane of polarisation of the light emitted by the laser is required to have an angle of 45° to the plane of the diagram.
• a quarter-wave plate 15 designed for use at the two wavelengths is inserted into one of the arms of the refractometer and this introduces a 90° phase delay between the 's' and 'p' polarised components of the laser light.
• the normal interfering 17 and non-interfering 19 output beams are used which are separated into their discrete wavelength components by a dispersive prism.
• the non-interfering output beams are incident on photodetectors P ⁇ and P 2 whilst the interfering output is separated into 's' and 'p' polarised components by a polarising beam splitter.
• the intensity of these polarised components is monitored by photodetectors P 3 to P 6 .
• This arrangement provides phase quadrature fringe detection and using the techniques described by PLM Heydemann in Applied Optics 20, 3382 (1981).
• the technique relies on calibrating the fringe detection electronics by modulating the optical path length in the refractometer in the same manner as that described previously. If a third wavelength is available an additional three photodetectors are required to measure the intensity of the non-interfering and interfering components.
• path length modulation is carried out every time fringe fraction data is required and in this way drift in the detection electronics is eliminated, whilst in the second embodiment an initial calibration of the interferometer signals allows fringe fractions to be determined for several hours without recalibration provided the laser intensity and the photodetector electronics are sufficiently stable.

Landscapes

• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10662775

Family Applications (1)

Country Status (4)

Families Citing this family (6)

Family Cites Families (4)

• 1989
  • 1989-09-08 GB GB898920366A patent/GB8920366D0/en active Pending
• 1990
  • 1990-09-07 EP EP19900913515 patent/EP0490957A1/de not_active Withdrawn
  • 1990-09-07 JP JP51263890A patent/JPH05500419A/ja active Pending
  • 1990-09-07 WO PCT/GB1990/001393 patent/WO1991003729A1/en not_active Application Discontinuation
  • 1990-09-07 GB GB9019648A patent/GB2236181B/en not_active Expired - Fee Related

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events