Patent Application
20230288182
The present invention relates to an optical complex amplitude measurement apparatus and an optical complex amplitude measurement method using a homodyne interferometer, which are applied to various fields such as biometric measurement and surface measurement.
In the various fields described above, a technique for measuring an optical complex amplitude including information related to both an intensity and a phase of an optical wave is used. Digital holography is a representative method in optical complex amplitude measurement techniques.
In digital holography, a signal beam including information of a measurement object is obtained, and the signal beam and a coherent reference beam are multiplexed. Then, an intensity distribution (interference fringe) caused by interference due to the multiplexing is acquired by a camera. By performing specific image processing on the acquired interference fringe by a calculator, an intensity distribution and a phase distribution (wave front) can be measured.
In measurement by digital holography, a measurement target may be at a long distance, such as an optical fiber between remote buildings or the atmosphere between buildings. In a case of measuring a measurement target, there is a method in which a signal beam from a light source is split into beams for two optical paths in advance and one of the beams is used as a reference beam. In this method, it is necessary to separately prepare an optical fiber for transmitting the reference beam and transmit the reference beam. As a result, a transmission cost increases. In addition, the optical fiber cannot be provided in circumstances where there are economical difficulties or physical difficulties.
For this reason, digital holography without using a reference beam has been proposed. In this digital holography, a signal beam from a light source is optically split, and then passes through a spatial filter to be described later. Thereby, a reference beam including plane wave components is generated.
An optical complex amplitude measurement apparatus 10 illustrated in
The laser 11 outputs (emits) a laser beam. The output laser beam passes through the measurement target 21, and thus a signal beam L11 is obtained. The signal beam L11 is input (incident) to the beam splitter 12. The beam splitter 12 splits the signal beam L11 by passing and reflecting the signal beam L11, outputs one split signal beam L11 to the spatial filter 13, and outputs the other split signal beam L11 to the mirror 15. The mirror 15 reflects the signal beam L11, and outputs the signal beam L11 to the beam splitter 16.
The spatial filter 13 extracts a plane wave component which is included in the signal beam L11 and in which a wave front is distorted due to the passage through the measurement target 21, and outputs a reference beam L12 including the extracted plane wave component. The output reference beam L12 is reflected by the mirror 14, and is output to the beam splitter 16.
The beam splitter 16 multiplexes the signal beam L11 and the reference beam L12, and outputs, to the camera 17, an interference fringe I1 that corresponds to an intensity distribution due to the multiplexing. The camera 17 acquires the interference fringe I1 and outputs interference fringe information I1a to the personal computer 18 via a conductive wire. The personal computer 18 calculates an intensity distribution and a phase distribution of the signal beam by performing specific image processing on the interference fringe information I1a (performing measurement processing).
This type of technique is described in Non Patent Literature 1.
However, in the optical complex amplitude measurement apparatus 10 described above, an optical power of the reference beam L12 obtained by the spatial filter 13 depends on an amount of the plane wave components included in the signal beam L11 passing through the measurement target 21. The spatial filter 13 extracts the plane wave component included in the signal beam L11. However, in a case where a distortion amount of the wave front in the signal beam L11 is changed, the amount of the plane wave components is changed, and as a result, the optical power of the reference beam L12 varies.
A relationship between the amount of the plane wave components of the signal beam L11 and the optical power of the reference beam L12 is indicated by a line E1 in
The present invention has been made in view of such circumstances, and an object of the present invention is to prevent a decrease in contrast of the interference fringe related to the signal beam passing through the measurement target and elimination of the interference fringe, and to measure an intensity distribution and a phase distribution of the interference fringe with high accuracy by the prevention.
In order to solve the above problems, there is provided an optical complex amplitude measurement apparatus including: a first light source that outputs a signal beam having a frequency f1; a second light source that outputs a signal beam having a frequency f2 as a reference beam; a polarization controller that performs a control of making a polarized beam of the signal beam which is output from the first light source and then passes through a measurement target match with a polarized beam of the reference beam output from the second light source; a spatial filter that extracts, from the matched signal beam related to the first light source, a plane wave component in which a wave front is distorted due to passage through the measurement target and outputs the signal beam which has the frequency f1 and includes the extracted plane wave component; a homodyne interferometer that multiplexes both the signal beam having the frequency f1 from the spatial filter and the reference beam having the frequency f2 from the second light source and inputs a beat signal due to a frequency difference (f1−f2) between the signal beam and the reference beam to a control end of the second light source; and a multiplexer that multiplexes the reference beam from the second light source and the signal beam from the polarization controller, wherein the second light source performs a phase synchronization control of the frequency of the reference beam to be output from the second light source such that the frequency difference (f1−f2) of the beat signal which is input to the control end becomes 0.
According to the present invention, it is possible to prevent a decrease in contrast of the interference fringe related to the signal beam passing through the measurement target and elimination of the interference fringe, and to measure an intensity distribution and a phase distribution of the interference fringe with high accuracy by the prevention.
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, in all the drawings in the present description, components having corresponding functions are denoted by the same reference numerals, and description thereof will be appropriately omitted.
<Configuration of First Embodiment>
An optical complex amplitude measurement apparatus 30 illustrated in
The first laser 31 constitutes a first light source according to the claims, and the second laser 32 constitutes a second light source according to the claims. The beam splitter 34b constitutes a multiplexer according to the claims. The personal computer 18 constitutes a calculator according to the claims.
As a feature of the first embodiment, the optical complex amplitude measurement apparatus 30 uses two light sources as the first and second lasers 31 and 32 that output (emit) laser beams having same frequencies f1 and f2. In a case where the frequencies f1 and f2 of the two laser beams do not match with each other, an interference fringe I1 which is input to the camera 17 becomes faded or is eliminated. As a result, the interference fringe I1 cannot be appropriately detected. For this reason, the frequency f2 of the laser beam from the second laser 32 is caused to match with the frequency f1 of the laser beam which is output from the first laser 31 and then passes through the measurement target 21, by using the homodyne interferometer 37.
The first laser 31 outputs a laser beam having a frequency f1. The output laser beam passes through the measurement target 21, and thus a signal beam L1 having the frequency f1 is obtained. The signal beam L1 is input (incident) to the polarization controller 33.
The polarization controller 33 performs control of making a polarized beam of the signal beam L1 match with a polarized beam of a reference beam L2 as a laser beam from the second laser 32. The matched signal beam L1 is input to the beam splitter 34a. Note that the reference beam L2 from the second laser 32 is input to the polarization controller 33 via a path of the beam splitters 34c and 37b, the spatial filter 36, and the beam splitter 34a, the reference beam L2 being input to the polarization controller 33.
The beam splitter 34a splits the signal beam L1 having the frequency f1 from the polarization controller 33 by passing and reflecting the signal beam L1, outputs one split signal beam L1 to the spatial filter 36, and outputs the other split signal beam L1 to the mirror 35. The mirror 35 reflects the signal beam L1, and outputs the signal beam L1 to the beam splitter 34b.
The spatial filter 36 extracts a plane wave component which is included in the signal beam L1 and in which a wave front is distorted due to the passage through the measurement target 21, and outputs the signal beam L1a which has the frequency f1 and includes the extracted plane wave component to the beam splitter 37b of the homodyne interferometer 37.
On the other hand, the second laser 32 outputs, as the reference beam L2, a laser beam having the same frequency f2 as the frequency f1. The output reference beam L2 is split by the beam splitter 34c. One split reference beam L2 is output to the beam splitter 34b. The other split reference beam L2 is output to the beam splitter 37b of the homodyne interferometer 37.
In the homodyne interferometer 37, the reference beam L2 is reflected by the beam splitter 37b. The reflected reference beam L2 is reflected by the mirror 37a, and is input to the beam splitter 37b again. In the beam splitter 37b, the reference beam L2 and the signal beam L1a from the spatial filter 36 are multiplexed. The photo diode 37c converts a beat beam due to a difference (also referred to as a frequency difference) between the frequencies f1 and f2 of the multiplexed signal beam L1a and the multiplexed reference beam L2 into a beat signal B1 serving as an electrical signal. The converted beat signal B1 is input to the control end of the second laser 32 via the conductive wire.
The second laser 32 performs a phase synchronization control of the frequency f2 of the reference beam L2 to be output from the second laser 32 such that a frequency difference (f1−f2) of the beat signal B1 becomes 0. The controlled reference beam L2 passes through the beam splitter 34c and then is fed back to the homodyne interferometer 37, while the controlled reference beam L2 is reflected by the beam splitter 34c and then is input to the other beam splitter 34b.
The beam splitter 34b multiplexes the signal beam L1 and the reference beam L2, and outputs, to the camera 17, an interference fringe I1 that corresponds to an intensity distribution by the multiplexing. The camera 17 acquires the interference fringe I1 and outputs interference fringe information I1a to the personal computer 18 via a conductive wire. The personal computer 18 calculates a light intensity and a phase of the signal beam L1 by performing specific image processing on interference fringe information I1a (performing measurement processing).
<Operation of First Embodiment>
Next, an optical complex amplitude measurement operation by the optical complex amplitude measurement apparatus 30 according to the first embodiment will be described with reference to flowcharts of
In step S1 illustrated in
In step S2, the polarization controller 33 performs a control such that the polarized beam of the signal beam L1 which is input matches with the polarized beam of the reference beam L2 which is output from the second laser 32, and the matched signal beam L1 is input to the beam splitter 34a.
In step S3, the beam splitter 34a splits the signal beam L1 having the frequency f1 from the polarization controller 33. One split signal beam L1 is output to the spatial filter 36, and the other split signal beam L1 is output to the mirror 35.
In step S4, the signal beam L1 is reflected by the mirror 35, and is output to the beam splitter 34b.
In step S5, the spatial filter 36 extracts a plane wave component which is included in the signal beam L1 and in which a wave front is distorted due to the passage of the measurement target 21. Further, the signal beam L1a which has the frequency f1 and includes the extracted plane wave component is output to the beam splitter 37b of the homodyne interferometer 37 from the spatial filter 36.
In step S6, the reference beam L2 having the frequency f2 is output from the second laser 32, and the reference beam L2 is split by the beam splitter 34c. One split reference beam L2 is output to the beam splitter 34b, and the other split reference beam L2 is output to the beam splitter 37b of the homodyne interferometer 37.
In step S7, the reference beam L2 is reflected by the beam splitter 37b of the homodyne interferometer 37. The reflected reference beam L2 is reflected by the mirror 37a, and is input to the beam splitter 37b again. In the beam splitter 37b, the reference beam L2 and the signal beam L1a from the spatial filter 36 are multiplexed. The beat signal B1 due to a frequency difference (f1−f2) between the multiplexed signal beam L1a and the multiplexed reference beam L2 is input from the photo diode 37c to the control end of the second laser 32 via the conductive wire.
Referring to
In step S9, the reference beam L2 controlled such that the frequency difference (f1−f2) becomes 0 is reflected by the beam splitter 34c, and then is input to the other beam splitter 34b.
In step S10, the signal beam L1 and the reference beam L2 are multiplexed by the beam splitter 34b, and the interference fringe I1 obtained by the multiplexing is output to the camera 17.
In step S11, the camera 17 acquires interference fringe information I1a by capturing the interference fringe I1, and outputs the acquired interference fringe information I1a to the personal computer 18.
In step S12, the personal computer 18 performs specific image processing of calculating the light intensity and the phase of the signal beam L1 from the interference fringe information I1a (performs measurement processing).
<Effects of First Embodiment>
Effects of the optical complex amplitude measurement apparatus 30 according to the first embodiment will be described.
(1a) The optical complex amplitude measurement apparatus 30 includes a first laser 31, a second laser 32, a polarization controller 33, a spatial filter 36, a homodyne interferometer 37, and a beam splitter 34b.
The first laser 31 outputs a signal beam L1 having a frequency f1. The second laser 32 outputs a signal beam L1 having a frequency f2 as a reference beam L2, and performs a phase synchronization control of making the frequency f2 match with the frequency f1. The polarization controller 33 performs a control of making a polarized beam of the signal beam L1, which is output from the first laser 31 and passes through a measurement target 21, match with a polarized beam of the reference beam L2 output from the second laser 32.
The spatial filter 36 extracts, from the matched signal beam L1 related to the first laser 31, a plane wave component in which a wave front is distorted due to the passage of the measurement target 21, and outputs the signal beam L1a which has the frequency f1 and includes the extracted plane wave component. The homodyne interferometer 37 multiplexes both the signal beam L1a having the frequency f1 from the spatial filter 36 and the reference beam L2 having the frequency f2 from the second laser 32, and inputs the beat signal B1 due to the frequency difference (f1−f2) between the signal beam L1a and the reference beam L2 to the control end of the second laser 32. The beam splitter 34b multiplexes the reference beam L2 from the second laser 32 and the signal beam L1 from the polarization controller 33. Further, the second laser 32 is configured to perform a phase synchronization control of the frequency of the reference beam L2 to be output from the second laser 32 such that the frequency difference (f1−f2) of the beat signal B1 to be input to the control end becomes 0.
According to this configuration, the polarized beam of the signal beam L1 which has the frequency f1 and is output from the first laser 31 is matched with the polarized beam of the reference beam L2 which has the frequency f2 and is output from the second laser 32. Further, the second laser 32 performs a phase synchronization control of the frequency of the reference beam L2 to be output from the second laser 32 such that the frequency difference (f1−f2) between the signal beam L1 having the frequency f1 and the signal beam L1 having the frequency f2 becomes 0. The signal beam L1 and the reference beam L2 when the frequency difference (f1−f2) becomes 0 by the control are multiplexed by the beam splitter 34b.
The reference beam L2 at this time is directly output from the second laser 32, and thus optical power can be stabilized. Further, the frequency difference (f1−f2) between the signal beam L1 which has the frequency f1 and is related to the first laser 31 and the reference beam L2 having the frequency f2 becomes 0 by the phase synchronization control. Therefore, an interference fringe obtained by multiplexing the signal beam L1 and the reference beam L2 by the beam splitter 34b has clear contrast. In other words, it is possible to prevent the contrast of the interference fringe from being decreased, and it is possible to prevent the interference fringe from being eliminated, the interference fringe being related to the signal beam L1 passing through the measurement target 21.
(2a) The camera 17 that captures an image of the interference fringe and obtains interference fringe information from the captured interference fringe is provided, the interference fringe being obtained by multiplexing the reference beam L2 from the second laser 32 and the signal beam L1 from the polarization controller 33 by the beam splitter 34b. In addition, there is further provided a personal computer 18 that measures an intensity distribution and a phase distribution of the signal beam L1 from the interference fringe information obtained by the camera 17.
According to this configuration, the reference beam L2 which is multiplexed with the signal beam L1 by the beam splitter 34b is directly output from the second laser 32. Thus, optical power can be stabilized. Therefore, the personal computer 18 calculates the interference fringe information obtained by capturing the interference fringe by the camera 17, the interference fringe being obtained by multiplexing the signal beam L1 and the reference beam L2 by the beam splitter 34b. Thereby, it is possible to measure the intensity distribution and the phase distribution of the signal beam L1 with high accuracy.
<Configuration of Second Embodiment>
The optical complex amplitude measurement apparatus 30A according to the second embodiment illustrated in
The homodyne interferometer 37A includes a mirror 37a, a beam splitter 37b, an optical fiber coupler 37d, a single-mode optical fiber 37e, and a photo diode 37c.
One end of the single-mode optical fiber 37e is connected to the beam splitter 37b via the optical fiber coupler 37d, and the other end of the single-mode optical fiber 37e is connected to the photo diode 37c. The single-mode optical fiber 37e performs plane wave component extraction processing which is the same processing as the processing of the spatial filter 36 (
The photo diode 37c converts a beat beam due to a frequency difference (f1−f2) between the multiplexed signal beam L1a and the multiplexed reference beam L2 into a beat signal B1 serving as an electrical signal, and inputs the beat signal B1 to the control end of the second laser 32 via the conductive wire.
The second laser 32 performs a phase synchronization control of the frequency f2 of the reference beam L2 to be output from the second laser 32 such that a frequency difference (f1−f2) of the beat signal B1 becomes 0. The controlled reference beam L2 passes through the beam splitter 34c and then is fed back to the homodyne interferometer 37A, while the controlled reference beam L2 is reflected by the beam splitter 34c and then is input to the other beam splitter 34b.
The signal beam L1 and the reference beam L2 are multiplexed by the beam splitter 34b, and the interference fringe I1 obtained by the multiplexing is output to the camera 17. The interference fringe information I1a is output from the camera 17 to the personal computer 18 via the conductive wire. The personal computer 18 executes calculation processing of an intensity distribution and a phase distribution of the signal beam L1 from the interference fringe information I1a.
<Effects of Second Embodiment>
Effects of the optical complex amplitude measurement apparatus 30A according to the second embodiment will be described.
(1b) The optical complex amplitude measurement apparatus 30A includes a first laser 31, a second laser 32, a polarization controller 33, a homodyne interferometer 37A, and a beam splitter 34b.
The first laser 31 outputs a signal beam L1 having a frequency f1. The second laser 32 outputs a signal beam L1 having a frequency f2 as a reference beam L2, and performs a phase synchronization control of the frequency f2 with respect to the frequency f1. The polarization controller 33 performs a control of making a polarized beam of the signal beam L1, which is output from the first laser 31 and passes through a measurement target 21, match with a polarized beam of the reference beam L2 output from the second laser 32.
The homodyne interferometer 37A includes the single-mode optical fiber 37e that extracts, from the matched signal beam L1 related to the first laser 31, a plane wave component in which a wave front is distorted due to the passage through the measurement target 21, multiplexes both the signal beam L1a which has the frequency f1 and includes the extracted plane wave component and the reference beam L2 having the frequency f2 from the second laser 32, and transmits a beam obtained by the multiplexing. Further, the homodyne interferometer 37A inputs, to the control end of the second laser 32, the beat signal B1 due to the frequency difference (f1−f2) between the signal beam L1a and the reference beam L2 from the single-mode optical fiber 37e. The beam splitter 34b multiplexes the reference beam L2 from the second laser 32 and the signal beam L1 from the polarization controller 33. Further, the second laser 32 is configured to perform a phase synchronization control of the frequency of the reference beam L2 to be output from the second laser 32 such that the frequency difference (f1−f2) of the beat signal B1 to be input to the control end becomes 0.
According to this configuration, the same functions and effects as those of the optical complex amplitude measurement apparatus 30 (
(2b) Similarly to the first embodiment, the optical complex amplitude measurement apparatus 30A according to the second embodiment further includes a camera 17 and a personal computer 18. According to this configuration, the same effects as those of the first embodiment can be obtained.
In actual measurement, it is necessary to appropriately change the first and second embodiments according to an algorithm used when calculating the intensity distribution and the phase distribution of the signal beam from the interference fringe information I1a. For example, in the optical complex amplitude measurement apparatuses 30 and 30A, there may be a case where a phase modulation element is inserted between the beam splitter 34c and the beam splitter 34b and a case where the beam splitter 34b is tilted and the reference beam L2 with an angle is multiplexed with the signal beam L1.
<Effects>
(1) There is provided an optical complex amplitude measurement apparatus including: a first light source that outputs a signal beam having a frequency f1; a second light source that outputs a signal beam having a frequency f2 as a reference beam; a polarization controller that performs a control of making a polarized beam of the signal beam which is output from the first light source and then passes through a measurement target match with a polarized beam of the reference beam output from the second light source; a spatial filter that extracts, from the matched signal beam related to the first light source, a plane wave component in which a wave front is distorted due to passage through the measurement target and outputs the signal beam which has the frequency f1 and includes the extracted plane wave component; a homodyne interferometer that multiplexes both the signal beam having the frequency f1 from the spatial filter and the reference beam having the frequency f2 from the second light source and inputs a beat signal due to a frequency difference (f1−f2) between the signal beam and the reference beam to a control end of the second light source; and a multiplexer that multiplexes the reference beam from the second light source and the signal beam from the polarization controller, in which the second light source performs a phase synchronization control of the frequency of the reference beam to be output from the second light source such that the frequency difference (f1−f2) of the beat signal which is input to the control end becomes 0.
According to this configuration, the polarized beam of the signal beam which has the frequency f1 and is output from the first light source is matched with the polarized beam of the reference beam which has the frequency f2 and is output from the second light source. Further, the second light source performs a phase synchronization control of the frequency of the reference beam to be output from the second light source such that the frequency difference (f1−f2) between the signal beam having the frequency f1 and the signal beam having the frequency f2 becomes 0. The signal beam and the reference beam when the frequency difference (f1−f2) becomes 0 by the control are multiplexed by the multiplexer.
The reference beam at this time is directly output from the second light source, and thus optical power can be stabilized. Further, the frequency difference (f1−f2) between the signal beam which has the frequency f1 and is related to the first light source and the reference beam having the frequency f2 becomes 0 by the phase synchronization control. Therefore, an interference fringe obtained by multiplexing the signal beam and the reference beam by the multiplexer has clear contrast. In other words, it is possible to prevent the contrast of the interference fringe from being decreased, and it is possible to prevent the interference fringe from being eliminated, the interference fringe being related to the signal beam passing through the measurement target.
(2) There is provided an optical complex amplitude measurement apparatus including: a first light source that outputs a signal beam having a frequency f1; a second light source that outputs a signal beam having a frequency f2 as a reference beam; a polarization controller that performs a control of making a polarized beam of the signal beam which is output from the first light source and then passes through a measurement target match with a polarized beam of the reference beam output from the second light source; a homodyne interferometer that includes a single-mode optical fiber, which extracts, from the matched signal beam related to the first light source, a plane wave component in which a wave front is distorted due to passage through the measurement target, multiplexes both the signal beam which has the frequency f1 and includes the extracted plane wave component and the reference beam having the frequency f2 from the second light source, and transmits a beam obtained by the multiplexing, and inputs a beat signal due to a frequency difference (f1−f2) between the signal beam and the reference beam to a control end of the second light source; and a multiplexer that multiplexes the reference beam from the second light source and the signal beam from the polarization controller, in which the second light source performs a phase synchronization control of the frequency of the reference beam to be output from the second light source such that the frequency difference (f1−f2) of the beat signal which is input to the control end becomes 0.
According to this configuration, the same functions and effect as those of the optical complex amplitude measurement apparatus according to Claim 1 can be obtained. On the other hand, as compared with the optical complex amplitude measurement apparatus, a spatial filter is not necessary. Thus, it is possible to reduce a size of the apparatus by that of the spatial filter.
(3) The optical complex amplitude measurement apparatus according to (1) or (2) further includes: a camera that captures an interference fringe obtained by multiplexing the reference beam after the phase synchronization control and the signal beam from the polarization controller by the multiplexer; and a calculator that performs image processing for calculating an intensity distribution and a phase distribution of the signal beam from interference fringe information obtained by the camera.
According to this configuration, the reference beam which is multiplexed with the signal beam by the multiplexer is directly output from the second light source. Thus, optical power can be stabilized. Therefore, the calculator performs image processing on the interference fringe information obtained by capturing the interference fringe by the camera, the interference fringe being obtained by multiplexing the signal beam and the reference beam by the multiplexer. Thereby, it is possible to measure the intensity distribution and the phase distribution of the signal beam with high accuracy.
In addition to the above, the specific configuration can be modified as appropriate, without departing from the scope of the present invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/029767 | 8/4/2020 | WO |
