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US 12,736,324 B2Granted
Interferometric displacement measurement apparatus
AssigneeOxford University Innovation Limited
InventorsArmin Reichold, Peter Qui, Songyuan Zhao, Edgar Brucke
Life of this patent
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Abstract
An interferometric displacement measurement apparatus (100) includes at least one measurement interferometer (103) for measuring a change in optical path difference between a measurement beam (150) and a reference beam. A light source module (118) is arranged to generate a modulated light beam, having a particular optical spectrum, from which the measurement beam and reference beam are derived. A data acquisition and analysis module (105) can determine a measure representative of the displacement using interference intensity data received from a photodetector (111) which detects the interference of the measurement beam with the reference beam.
Claims19
An interferometric displacement measurement apparatus, comprising:
- at least one measurement interferometer for measuring a change in optical path difference between a measurement beam and a reference beam, the reference beam and the measurement beam having an optical path difference determined by a longitudinal dimension of the measurement interferometer, said change in optical path difference being caused by a respective displacement, wherein the at least one measurement interferometer comprises a photodetector for detecting the interference of the measurement beam with the reference beam;
- a data acquisition and analysis module arranged to receive interference intensity data from the photodetector;
- and
- a light source module comprising:
- a light source arranged to output light having a substantially constant optical carrier frequency;
- and
- an electro-optic modulator arranged to modulate the light output from the light source;
- wherein the light source module is arranged to generate a modulated light beam, from which the measurement beam and reference beam are derived;
- and
- wherein an optical spectrum of the modulated light beam comprises:
- a first set of equally spaced frequency peaks having a first frequency spacing, wherein the number, position and power of frequency peaks in the first set are a function of a first modulation frequency and a first modulation depth, and wherein the first set is centred on a central frequency;
- and
- a second set of equally spaced frequency peaks having a second frequency spacing, wherein the number, position and power of frequency peaks in the second set are a function of a second modulation frequency and a second modulation depth, and wherein the second set is centred on the same central frequency as the first set;
- wherein the first frequency spacing is different to the second frequency spacing and the frequency peaks of the first and second sets are phase locked against each other;
- wherein the electro-optic modulator is arranged to be driven by a phase modulation voltage that causes the electro-optic modulator to generate the optical spectrum from the light source, wherein the phase modulation voltage is a time dependent function of the first modulation frequency, the second modulation frequency, the first modulation depth and the second modulation depth, the first modulation frequency determining the first frequency spacing and the second modulation frequency determining the second frequency spacing;
- and
- wherein the first frequency spacing (Δv1) and second frequency spacing (Δv2) ae such that
- k
-
- Δ
-
- v
- 1
- Δ
-
- v
- 2
- ≠
- n
- for k>106 where k and n are integers;
- wherein the data acquisition and analysis module is arranged to determine a measure representative of the displacement using the interference intensity data received from the photodetector.
28 more elements
An interferometric displacement measurement apparatus, comprising:
- at least one measurement interferometer for measuring a change in optical path difference between a measurement beam and a reference beam, the reference beam and the measurement beam having an optical path difference determined by a longitudinal dimension of the measurement interferometer, said change in optical path difference being caused by a respective displacement, wherein the at least one measurement interferometer comprises a photodetector for detecting the interference of the measurement beam with the reference beam;
- a data acquisition and analysis module arranged to receive interference intensity data from the photodetector;
- and
- a light source module comprising:
- a light source arranged to output light having a substantially constant optical carrier frequency;
- and
- an electro-optic modulator arranged to modulate the light output from the light source;
- wherein the light source module is arranged to generate a modulated light beam, from which the measurement beam and reference beam are derived;
- and
- wherein an optical spectrum of the modulated light beam comprises:
- a first set of equally spaced frequency peaks having a first frequency spacing, wherein the number, position and power of frequency peaks in the first set are a function of a first modulation frequency and a first modulation depth, and wherein the first set is centred on a central frequency;
- and
- a second set of equally spaced frequency peaks having a second frequency spacing, wherein the number, position and power of frequency peaks in the second set are a function of a second modulation frequency and a second modulation depth, and wherein the second set is centred on the same central frequency as the first set;
- wherein the first frequency spacing is different to the second frequency spacing and the frequency peaks of the first and second sets are phase locked against each other;
- wherein the electro-optic modulator is arranged to be driven by a phase modulation voltage that causes the electro-optic modulator to generate the optical spectrum from the light source, wherein the phase modulation voltage is a time dependent function of the first modulation frequency, the second modulation frequency, the first modulation depth and the second modulation depth, the first modulation frequency determining the first frequency spacing and the second modulation frequency determining the second frequency spacing;
- wherein the data acquisition and analysis module is arranged to determine a measure representative of the displacement using the interference intensity data received from the photodetector;
- and
- wherein the data acquisition and analysis module is arranged to store a modulation scheme, wherein the modulation scheme is used to generate the phase modulation voltage;
- wherein the modulation scheme comprises the first modulation frequency, the second modulation frequency, the first modulation depth and the second modulation depth;
- wherein the interferometric displacement measurement apparatus is arranged to obtain interference intensity data arising from an interference between the measurement beam and the reference beam, the interference intensity data comprising at least one odd intensity band and at least one even intensity band wherein the odd interference intensity frequency bands and even interference intensity frequency bands occur at frequencies given by fj,h=jfm1+hfm2, wherein fm1 is the first modulation frequency, fm2 is the second modulation frequency, j and h each comprise an integer >0;
- and wherein an intensity band is an odd intensity band when the sum of j and h is an odd whole number and wherein an intensity band is an even intensity band when the sum of j and h is an even whole number wherein the apparatus is arranged to:
- determine a modulation frequency pair comprising the first modulation frequency and the second modulation frequency;
- given said modulation frequency pair, determine a modulation depth pair comprising the first modulation depth and the second modulation depth, by:
- for each of a plurality of modulation depth pairs, wherein the modulation depth of the first modulation depth is between 0 and 6 and the modulation depth of the second modulation depth is between 0 and 6:
- for each of a plurality of optical path differences:
- evaluate the plurality of modulation depth pairs;
- determine a first measure representative of the sum of the power of the odd interference intensity frequency bands and a second measure representative of the sum of the power of the even interference intensity frequency bands;
- for each of the plurality of modulation depth pairs:
- determine the lowest first measure representative of the sum of the power of the odd interference intensity frequency bands and the lowest second measure representative of the sum of the power of the even interference intensity frequency bands from the plurality of optical path differences;
- and
- take the lower of the lowest first measure representative of the sum of the power of the odd interference intensity frequency bands and the lowest second measure representative of the sum of the power of the even interference intensity frequency bands, and
- select one of the plurality of modulation depth pairs, wherein the selected modulation depth pair gives the largest value of said lower measure.
29 more elements
An interferometric displacement measurement apparatus, comprising:
- at least one measurement interferometer for measuring a change in optical path difference between a measurement beam and a reference beam, the reference beam and the measurement beam having an optical path difference determined by a longitudinal dimension of the measurement interferometer, said change in optical path difference being caused by a respective displacement, wherein the at least one measurement interferometer comprises a photodetector for detecting the interference of the measurement beam with the reference beam;
- a data acquisition and analysis module arranged to receive interference intensity data from the photodetector;
- and
- light source module comprising:
- a light source arranged to output light having a substantially constant optical carrier frequency;
- and
- an electro-optic modulator arranged to modulate the light output from the light source;
- wherein the light source module is arranged to generate a modulated light beam, from which the measurement beam and reference beam are derived;
- and
- wherein an optical spectrum of the modulated light beam comprises:
- a first set of equally spaced frequency peaks having a first frequency spacing, wherein the number, position and power of frequency peaks in the first set are a function of a first modulation frequency and a first modulation depth, and wherein the first set is centred on a central frequency;
- and
- a second set of equally spaced frequency peaks having a second frequency spacing, wherein the number, position and power of frequency peaks in the second set are a function of a second modulation frequency and a second modulation depth, and wherein the second set is centred on the same central frequency as the first set;
- wherein the first frequency spacing is different to the second frequency spacing and the frequency peaks of the first and second sets are phase locked against each other;
- wherein the electro-optic modulator is arranged to be driven by a phase modulation voltage that causes the electro-optic modulator to generate the optical spectrum from the light source, wherein the phase modulation voltage is a time dependent function of the first modulation frequency, the second modulation frequency, the first modulation depth and the second modulation depth, the first modulation frequency determining the first frequency spacing and the second modulation frequency determining the second frequency spacing;
- wherein the data acquisition and analysis module is arranged to determine a measure representative of the displacement using the interference intensity data received from the photodetector;
- and
- wherein the data acquisition and analysis module is further arranged to carry out an analysis algorithm for determining a change in optical path difference from a time-varying interference intensity signal, the analysis algorithm comprising:
- receiving a time-varying interference intensity signal, derived from interference detected at a photodetector, comprising a plurality of odd interference intensity frequency bands and a plurality of even interference intensity frequency bands;
- extracting each odd interference intensity frequency band using a respective lock-in amplifier;
- summing and normalising the plurality of odd interference intensity frequency bands;
- extracting each even interference intensity frequency band using a respective lock-in amplifier;
- summing and normalising the plurality of even interference intensity frequency bands output from the lock-in amplifiers;
- generating an interferometric quadrature signal by combining the normalised sum of the odd interference intensity frequency bands and the normalised sum of the even interference intensity frequency bands;
- and
- calculating the change in optical path difference from said interferometric quadrature signal.
23 more elements
A method of determining a modulation scheme for use in an interferometric displacement measurement apparatus;
- wherein the modulation scheme is used to generate a time-varying phase modulation voltage for modulating a light beam from which a measurement beam and a reference beam are derived;
- wherein the modulation scheme comprises a first modulation frequency, a second modulation frequency, a first modulation depth and a second modulation depth;
- wherein the interferometric displacement measurement apparatus is arranged to obtain interference intensity data arising from an interference between the measurement beam and the reference beam, the interference intensity data comprising at least one odd band and at least one even band wherein the odd interference intensity frequency bands and even interference intensity frequency bands occur at frequencies given by fj,h=jfm1+hfm2, wherein fm1 is the first modulation frequency, fm2 is the second modulation frequency, j and h each comprise an integer >0;
- and wherein a band is an odd band when the sum of j and h is an odd whole number and wherein a band is an even band when the sum of j and h is an even whole number;
- the method comprising:
- determining a modulation frequency pair comprising the first modulation frequency and the second modulation frequency;
- given said modulation frequency pair, determining a modulation depth pair comprising the first modulation depth and the second modulation depth, by:
- for each of a plurality of modulation depth pairs, wherein the modulation depth of the first modulation depth is between 0 and 6 and the modulation depth of the second modulation depth is between 0 and 6:
- for each of a plurality of optical path differences:
- evaluating the plurality of modulation depth pairs;
- determining a first measure representative of the sum of the power of the odd interference intensity frequency bands and a second measure representative of the sum of the power of the even interference intensity frequency bands;
- for each of the plurality of modulation depth pairs:
- determining the lowest first measure representative of the sum of the power of the odd interference intensity frequency bands and the lowest second measure representative of the sum of the power of the even interference intensity frequency bands from the plurality of optical path differences;
- and
- taking the lower of the lowest first measure representative of the sum of the power of the odd interference intensity frequency bands and the lowest second measure representative of the sum of the power of the even interference intensity frequency bands, and
- selecting one of the plurality of modulation depth pairs, wherein the selected modulation depth pair gives the largest value of said lower measure.
13 more elements
Dependencies are inferred from the claim text.