Method for imaging or spectroscopy with a non-linear interferometer

Abstract

A system and method is provided for imaging and/or spectroscopy involving generation of a first signal field and a first idler field, illumination of the object with the first idler field, generation of second signal field and a second idler field, combination of the first and second idler fields, such that the two fields are indistinguishable, combination of the first and second signal fields, such that the two fields interfere, first measurement of the interfered signal field by a detection means, one or more additional measurements of the interfered signal field, wherein for each additional measurement a different phase shift is generated in the setup, and wherein all measurements are carried out within the stability time of the setup, and calculation of a phase function.

Claims

1. Method for imaging and/or spectroscopy using an interferometer setup, comprising the steps i) generation of a first signal field s.sub.1 and a first idler field i.sub.1, by pumping a first non-linear medium with a pump beam, such that the two fields are correlated, ii) illumination of an object with the first idler i.sub.1 field, respectively by transmission and/or reflection, iii) generation of a second signal field s.sub.2 and a second idler field i.sub.2, by pumping a spatial separate second non-linear medium with the pump beam, or by pumping the first non-linear medium a second time with the pump beam, such that the two fields are correlated, iv) combination of the first i.sub.1 and second i.sub.2 idler fields, such that the two fields are indistinguishable, and combination of the first s.sub.1 and second s.sub.2 signal fields, such that the two fields interfere, v) first measurement of the interfered signal field s.sub.12 by a detection means, vi) one or more additional measurements of the interfered signal field s.sub.12 by the detection means, wherein for each additional measurement in step vi) a phase shift α is generated in the interferometer setup, and the phase shift α is varied from measurement-to-measurement in a known manner, and wherein the first measurement in step v) and the one or more additional measurements in step vi) are all carried out within a stability time of the interferometer setup, and vii) calculation of a phase function Φ of the object out of the measurements from step v) and step vi) in order to get an image and/or a spectrum of the object.

2. The method according to claim 1, wherein the phase shift α is created in the first signal field s.sub.1 and/or in the second signal field s.sub.2 and/or in the first idler field i.sub.1 and/or in the second idler field i.sub.2 and/or in the pump beam in front of the first non-linear medium, preferably in the first pump beam and/or in the pump beam in front of the second non-linear medium, preferably in the second pump beam and/or between the first and second signal fields i.sub.1 and i.sub.2 and/or between the first signal and idler fields s.sub.1 and i.sub.1, and/or between the first and the second pump beams, and/or in the interfered signal field s.sub.12.

3. The method according to claim 1, wherein the phase shift α is created by changing the path length of one or more field/s, and/or changing the wavelength of the first and/or second pump beam, and/or by thermal effects, and/or by spatial displacement or change of the optical path length in one or both interferometer arms.

4. The method according to claim 1, wherein the phase shifts can be introduced by a translation of a mirror and/or a translation of an optical surface and/or a translation of a dichroic mirror, respectively movable by a piezo element, and/or by a fiber expander, and/or by tilting a plane-parallel plate, and/or by an optical frequency difference between two beams, preferably two pump beams and/or by the change of the polarization by an EOM and/or wave plates and/or a polarizing beam splitter and/or a polarizer, and/or by tilting a plane-parallel plate, and or by a rotation or movement of a birefringent plate.

5. The method according to claim 1, wherein the phase of the setup in step v) is unknown and/or arbitrary.

6. The method according to claim 1, wherein in step i) and/or step iii) the signal and idler fields are separated by a separation means in or behind the crystal or are separated due to the generation of the signal and idler fields in the non-linear medium, respectively separated due to the generation of the fields in a BBO crystal.

7. The method according to claim 1, wherein in step v) and/or vi) the constructive and destructive interference is measured, respectively by a first and second detection means behind two output arms of an interference means, respectively a 50/50 beam splitter, respectively wherein the 50/50 beam splitter is the signal combining means.

8. The method according to claim 1, wherein in step v) and/or vi) one detection means is used, wherein for each measurement a phase shift α is generated, or two or more detection means are used, wherein for each additional detection means the same or a separate phase shift α is generated.

9. Apparatus for imaging and/or spectroscopy using an interferometer setup, comprising a pump source to generate a pump beam, and a first signal s.sub.1 and idler i.sub.1 field generation means pumped by the pump beam, and a second signal s.sub.2 and idler i.sub.2 field generation means pumped by the pump beam, wherein the first and the second field generation means are two spatial separated non-linear media pumped by the pump beam, or one non-linear medium, pumped by the pump beam a first time to generate a first signal s.sub.1 and idler i.sub.1 field and pumped a second time to generate a second signal s.sub.2 and idler i.sub.2 field, and an object to be measured which is illuminated, respectively by transmission or reflection, by the first idler field i.sub.1, and a signal combining means to overlap the first signal s.sub.1 and second signal s.sub.2 fields, such that the two fields interfere, and an idler combining means to overlap the first idler i.sub.1 and second idler i.sub.2 fields, such that the two fields are indistinguishable, and a detection means to detect the intensity and/or phase of the interfered signal field, characterized in that, a phase shifter is arranged in the first signal s.sub.1 field, and/or in the second signal s.sub.2 field, and/or in the pump beam, and/or in the indistinguishable first and second signal fields s.sub.1 and s.sub.2, wherein the phase shifter is adapted to introduce a phase shift α in the apparatus during the measurement in order to get an image and/or a spectrum of the object, and the phase shift α is varied during the measurement in a known manner.

10. Apparatus according to claim 9, wherein the phase shifts can be introduced by a movable mirror and/or a movable optical surface and/or a movable dichroic mirror, respectively movable by a piezo element, and/or by a fiber expander, and/or by a tiltable plane-parallel plate, and/or by an optical frequency difference between two beams, preferably two pump beams and/or by the change of the polarization by an EOM and/or wave plates and/or a polarizing beam splitter and/or a polarizer, and/or by a tiltable plane-parallel plate, and/or by a rotatable or moveable birefringent plate.

11. Apparatus according to claim 9, wherein the separation means and the signal combining means are arranged in a Mach-Zehnder interferometer configuration or a laser Fizeau interferometer configuration, or a Michelson-interferometer configuration.

12. Apparatus according to claim 11, wherein the phase shift a is created within the interferometer.

13. Apparatus according to claim 9, wherein the apparatus comprises a control device adapted to provide one of the methods above, wherein the control device is connected to the phase shifter and to the detection means.

14. A non-transient computer-readable medium having stored thereon a computer-readable program for executing a method for at least one of imaging or spectroscopy using an interferometer setup, the method comprising the steps of: i) generation of a first signal field s.sub.1 and a first idler field i.sub.1, by pumping a first non-linear medium with a pump beam, such that the two fields are correlated, ii) illumination of an object with the first idler i.sub.1 field, respectively by transmission and/or reflection, iii) generation of a second signal field s.sub.2 and a second idler field i.sub.2, by pumping a spatial separate second non-linear medium with the pump beam, or by pumping the first non-linear medium a second time with the pump beam, such that the two fields are correlated, iv) combination of the first i.sub.1 and second i.sub.2 idler fields, such that the two fields are indistinguishable, and combination of the first s.sub.1 and second s.sub.2 signal fields, such that the two fields interfere, v) first measurement of the interfered signal field s.sub.12 by a detection means, vi) one or more additional measurements of the interfered signal field sit by the detection means, wherein for each additional measurement in step vi) a phase shift α is generated in the interferometer setup, and the phase shift α is varied from measurement-to-measurement in a known manner, and wherein the first measurement in step v) and the one or more additional measurements in step vi) are all carried out within a stability time of the interferometer setup, and vii) calculation of a phase function Φ of the object out of the measurements from step v) and step vi) in order to get an image and/or a spectrum of the object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

(2) FIG. 1: A first embodiment of the apparatus of the invention in a Mach-Zehnder-Interferometer setup;

(3) FIG. 2: A second embodiment of the apparatus of the invention in a Fizeau-Interferometer setup;

(4) FIG. 3: A third embodiment of the apparatus of the invention;

(5) FIG. 4: A fourth embodiment of the apparatus of the invention in a Mach-Zehnder-Interferometer setup with BBO-crystals.

DETAILED DESCRIPTION

(6) FIG. 1 shows a first embodiment of the apparatus of the invention in a Mach-Zehnder-Interferometer configuration. A pump source 2 emits a pump beam 3, preferably a laser beam. The pump beam 3 is split on a pump beam separation means, here a pump beam splitter 6 splits the pump beam 3 into two coherent pump beams 3a and 3b. The pump beam splitter 6 can be a normal beam splitter or a polarizing beam splitter with an additional wave plate (not shown) to rotate the polarization of one of the pump beams 3a or 3b in order to pump a first and second field generation means 10 or 20. The polarization of the first and/or the second pump beams 3a and 3b can be rotated by a wave plate (not shown) to enable the pump of the first and second field generation means 10 and 20. For that, the first pump beam 3a is guided into the first generation means 10 and the second pump beam 3b is guided by a first mirror 4a into the second generation means 20. In the path of the second pump beam 3b a third dichroic mirror 5c is arranged allowing the second pump beam 3b to travel through the third dichroic mirror 5c.

(7) As described above, the first generation means 10 is pumped by the first pump beam 3a, generating a first signal field 11 and first idler field 12. The first pump beam 3a is blocked after the first generation means 10 by a first dichroic mirror 5a allowing the first signal field 11 and first idler field 12 to pass through.

(8) The first signal field 11 and first idler field 12 are then separated on a separation means, in FIG. 1 on a second dichroic mirror 5b. The first signal field 11 is transmitted through the second dichroic mirror 5b and travels guided by second mirror 4b, preferably by a movable mirror to a beam splitter 7.

(9) The first idler field 12 is reflected on the second dichroic mirror 5b and interacts by transmission and/or reflection with an object 30. The first idler field 12 is then guided by the third dichroic mirror 5c into the second generation means 20.

(10) The second generation means 20 is pumped by the second pump beam 3b, generating a second signal field 21 and second idler field 22. In and/or behind the second generation means 20 the first idler field 12 overlaps spatially with the second idler field 22, such that the two fields are indistinguishable. The second pump beam 3b and the overlapping first and second idler fields 12 and 22 are blocked by a fourth dichroic mirror 5d whereas the second signal field 21 is transmitted and guided on the beam splitter 7.

(11) A Mach-Zehnder-Interferometer is created with an upper and a lower interferometer-arm between the pump beam splitter 6 and the beam splitter 7.

(12) The first and second signal fields 11 and 21 interfere on the beam splitter 7. On both outputs of the beam splitter 7, detection means, preferably CCD cameras are arranged to detect the intensity pattern of the interfered signal field 40.

(13) The phase shift α is generated by one or more phase shifting means 50. In FIG. 1 more phase shifting means 50 are depicted whereas only one but also more phase shifting means 50 can be arranged in the apparatus for imaging and spectroscopy 1. In FIG. 1, only as an example, the phase shifting means 50 are shown on more places. The phase shifting means 50 can be the second mirror 4b by translation of the mirror preferably movable by a piezo element. The phase shifting means 50 can also be a fiber expander (for a fiber interferometer setup), and/or a tilting plane-parallel plate, and/or an optical element introducing a frequency difference between the two pump beams 3a and 3b, and/or an EOM (electro optic modulator) and/or wave plates and/or a polarizing beam splitter and/or a polarizer by changing the polarization, and/or plane-parallel plate by tilting the plate, and or a birefringent plate by tilting the plate.

(14) The possible places of the phase shifting means 50 are in the first signal field s.sub.1 11 and/or, in the second signal field s.sub.2 21 and/or, in the first idler field i.sub.1 12 and/or, in the second idler field i.sub.2 22 and/or, in the first pump beam 3a and/or, in the second pump beam 3b.

(15) FIG. 2 shows a second embodiment of the apparatus of the invention in a Fizeau-Interferometer configuration. The pump source 2 emits a pump beam 3 which is transmitted through a first dichroic mirror 5a and pumps a generation means 10, wherein the generation means 10 acts in this pump direction as a first generation means 10 and generates the first signal field 11 and the first idler field 12.

(16) The first signal field 11 and the pump beam 3 transmit through a second dichroic mirror 5b and are reflected on the first mirror 4a, preferably on the movable first mirror 4a. After the reflection on the first mirror 4a the signal field 11 and the pump beam 3 are transmitted through the dichroic mirror 5b a second time and are guided into the generation means 10.

(17) The first idler field 12 is reflected on the second dichroic mirror 5b and interacts with the object 30 by transmission and/or reflection and is guided by a second mirror 4b back on the second dichroic mirror 5b and is guided into the generation means 10.

(18) By the second time, the generation means is pumped from the right side in FIG. 2 by the pump beam 3, the generation means 10 acts as a second generation means by this pump direction generating the second signal field 21 and second idler field 22. The first signal field 11 and the second signal field 21 overlap spatially in and behind the generation means 10, such that the signal fields interfere. The first idler field 12 and the second idler field 22 overlap spatially in and behind the generation means, such that the paths of the first idler field 12 and the second idler field 22 are indistinguishable. The interfered signal fields 40 are reflected on the first dichroic mirror and detected by the detection means.

(19) As in FIG. 1, here in FIG. 2 more phase shifting means 50 are depicted whereas only one but also more phase shifting means 50 can be arranged in the apparatus for imaging and spectroscopy 1. In FIG. 2, only as an example, the phase shifting means 50 are shown on more places. The phase shifting means 50 can be the first mirror 4a by translation of the mirror, preferably movable by a piezo element. The phase shifting means 50 can be the second mirror 4b by translation of the mirror (not shown in FIG. 2), preferably movable by a piezo element. The phase shifting means 50 can also be a fiber expander (for a fiber interferometer setup), and/or a tilting plane-parallel plate, and/or an optical element introducing a frequency difference between the two pump beams 3a and 3b, and/or an EOM (electro optic modulator) and/or wave plates and/or a polarizing beam splitter and/or a polarizer by changing the polarization, and/or plane-parallel plate by tilting the plate, and or a birefringent plate by tilting the plate.

(20) The possible places of the phase shifting means 50 are in the first signal field s.sub.1 11 and/or in the first idler field i.sub.1 12 and/or in the pump beam 3.

(21) FIG. 3 shows a third embodiment of the apparatus of the invention in a Fizeau-Interferometer-like configuration. The difference to FIG. 2 is that in FIG. 3 the first dichroic mirror 5a is replaced by a beam splitter 7, and the first signal field 11 is reflected on the surface of the generation means 10 to change the pump direction for the generation of the second signal 21 and idler 22 fields. Only the first idler beam is transmitted through the surface, interacts with the object and is guided back in the generation means 10.

(22) FIG. 4 shows a fourth embodiment of the apparatus of the invention in a Mach-Zehnder-Interferometer setup with BBO-crystals.

(23) The pump source 2 emits a pump beam 3 which is split into a first and a second pump beam 3a and 3b, whereas the polarization can be adjusted by a wave plate (not shown) to pump the first and second generation means 10 and 20. In this embodiment, the first and second generation means 10 and 20 are BBO crystals, emitting the first signal field 11 and the first idler field 21 in the first generation means 10, and the second signal field 21 and the second idler field 22 in the second generation means 20 under a specific angle.

(24) The first generation means 10 pumped by the first pump beam 3a generates the first signal field 11 and the first idler field 21. The first signal field 11 is reflected on the first mirror 4a, preferably on a movable mirror and guides the first signal field 11 into the beam splitter 7. The first idler field 12 interacts with the object 30 by transmission and/or reflection and is guided into the second generation means 20.

(25) The second generation means 20 pumped by the second pump beam 3b generates the second signal field 21 and the second idler field 22. The first and second idler fields 12 and 22 overlap spatially in and behind the second generation means 20 such that the first and second idler fields 12 and 22 are indistinguishable. The second signal field 21 is guided to the beam splitter 7 and interferes with the first signal field 11 on the beam splitter. On both output arms of the beam splitter, detection means 8 are arranged to detect the interfered signal fields 40.

(26) As before, here in FIG. 4 more phase shifting means 50 are depicted whereas the only one or more phase shifting means 50 can be arranged in the apparatus for imaging and spectroscopy 1. In FIG. 4, only as an example, the phase shifting means 50 are shown on more places. The phase shifting means can be the first mirror 4a by translation of the mirror, preferably movable by a piezo element. The phase shifting means 50 can also be a fiber expander (for a fiber interferometer setup), and/or a tilting plane-parallel plate, and/or an optical element introducing a frequency difference between the two pump beams 3a and 3b, and/or an EOM (electro optic modulator) and/or wave plates and/or a polarizing beam splitter and/or a polarizer by changing the polarization, and/or plane-parallel plate by tilting the plate, and or a birefringent plate by tilting the plate.

(27) The possible places of the phase shifting means 50 are in the first signal field s.sub.1 11 and/or in the second signal field s.sub.2 21 and/or in the first idler field i.sub.1 12 and/or, in the second idler field i.sub.2 22 and/or in the first pump beam 3a and/or in the second pump beam 3b.

REFERENCE SYMBOL LIST

(28) 1 apparatus for imaging and spectroscopy 2 pump source 3 pump beam 3a first pump beam 3b second pump beam 4a first mirror 4b second mirror 5a first dichroic mirror 5b second dichroic mirror 5c third dichroic mirror 5d fourth dichroic mirror 6 pump beam splitter 7 beam splitter (BS) 8 detection means 10 first field generation means 11 first signal field 12 first idler field 20 second field generation means 21 second signal field 22 second idler field 30 object 40 interfered signal field 50 phase shifter