# US 7,609,392 B2 · Harmonically matched diffraction grating pair

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- Number: US 7,609,392 B2
- Document ID: US7609392B2
- Status: Granted
- Kind: B2
- Application number: 11/770,553
- Filed: 2007-06-28
- Published: 2008-01-03
- Granted: 2009-10-27
- Earliest claimed: 2006-06-28
- Assignees: California Institute of Technology
- Inventors: Zahid Yaqoob; Jigang Wu; Marinko Sarunic; Changhuei Yang
- Web page: https://patentgrep.com/patent/US7609392B2

## CPC

- G01B 9/0201: {using temporal phase variation}
- G01B 9/02004: using frequency scans
- G01B 9/02081: {simultaneous quadrature detection, e.g. by spatial phase shifting}
- G01B 9/0209: Low-coherence interferometers
- G01B 2290/30: Grating as beam-splitter
- G01B 2290/45: Multiple detectors for detecting interferometer signals
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS

## Abstract

A method and device realize shallow gratings-based planar beam splitter/combiner. Non-trivial phase shifts between different ports of resulting interferometers are used to acquire full-field phase measurements. The non-trivial phase shifts between different ports of the planar beam splitter/combiner can be adjusted by simply shearing one grating with respect to the second grating. The two shallow diffraction gratings are harmonically-related and can be recorded on a single substrate for compact interferometric based schemes. During the recording process, the two gratings are aligned such that the grating planes and the grating vectors are parallel to that of each other. The relative phase of the recording beams controls the shearing between the recorded harmonically-related shallow phase gratings. The relative shearing of the two gratings defines the non-trivial phase shift between different ports of the compact planar beam splitter/combiner.

## Claims

### Claim 1

A beam splitter/combiner used to acquire phase measurements, comprising:
a first shallow diffraction grating and a second shallow diffraction grating that are harmonically-related, wherein:
the first grating and the second grating are aligned such that grating planes and grating vectors of the first and second gratings are parallel to that of each other; and
a phase shift at two or more output ports of a resulting interferometer is adjustable by shearing the first grating with respect to the second grating; and
the phase shift is non-trivial.

### Claim 2 (depends on 1)

The beam splitter/combiner of claim 1, wherein:
the beam splitter/combiner is utilized in a Michelson interferometer; and
the beam splitter/combiner produces interference signals at three output ports of the interferometer, wherein the phase shift between at least two of the three output ports is non-trivial.

### Claim 3 (depends on 1)

The beam splitter/combiner of claim 1, wherein the first and second diffraction gratings are holographically recorded simultaneously on a single substrate.

### Claim 4 (depends on 1)

The beam splitter/combiner of claim 1, wherein the beam splitter/combiner is utilized in a Mach-Zehnder interferometer for full field phase imaging of a phase object, the Mach-Zehnder interferometer comprising:
a lens L1 configured to collimate a filtered laser beam; and
a pair of lenses that create a microscope system that images the phase object onto two CCD (charge coupled device) cameras.

### Claim 5 (depends on 4)

The beam splitter/combiner of claim 4, wherein the phase object comprises two bars of PMMA (polymethyl methacrylate) material on a glass plate.

### Claim 6 (depends on 4)

The beam splitter/combiner of claim 4, wherein the phase object is a microfluidic channel and the Mach-Zehnder interferometer is utilized to observe liquid diffusion in the microfluidic channel.

### Claim 7 (depends on 1)

The beam splitter/combiner of claim 1, wherein:
the beam splitter/combiner is utilized in a spectrometer-based optical coherence tomography (OCT) system;
the beam splitter/combiner is used to split a collimated beam into a sample and reference beam;
a spherical lens L1 focuses an incident light beam and collects back scattered light from a sample;
a spherical lens L2 collects angularly dispersed reference light;
the back scattered light and the reference light undergo a diffraction at the beam splitter/combiner where the back scattered light and the reference light are split and combined at output ports; and
lenses L3 and L4 collect and focus light from the output ports onto CCD (charge coupled device) arrays.

### Claim 8

A method for fabricating a beam splitter/combiner used for acquiring phase measurements comprising:
recording a first shallow diffraction grating and a second shallow diffraction grating on a substrate, wherein:
the first grating and the second grating are harmonically related;
the first grating and the second grating are aligned such that grating planes and grating vectors of the first and second gratings are parallel to that of each other; and
a phase shift at two or more output ports of a resulting interferometer is adjustable by shearing the first grating with respect to the second grating; and
the phase shift is non-trivial.

### Claim 9 (depends on 8)

The method of claim 8, wherein the first and second gratings constitute a beam splitter/combiner, the method further comprising:
utilizing the beam splitter/combiner in a Michelson interferometer; and
the beam splitter/combiner producing interference signals at three output ports of the interferometer, wherein the phase shift between at least two of the three output ports is non-trivial.

### Claim 10 (depends on 8)

The method of claim 8, wherein the first and second diffraction gratings are holographically recorded simultaneously on a single substrate.

### Claim 11 (depends on 8)

The method of claim 8, wherein the first and second gratings constitute a beam splitter/combiner, the method further comprising:
conduct a full field phase imaging of a phase object utilizing the beam splitter/combiner in a Mach-Zehnder interferometer, the Mach-Zehnder interferometer comprising:
a lens L1 configured to collimate a filtered laser beam; and
a pair of lenses that create a microscope system that images the phase object onto two CCD (charge coupled device) cameras.

### Claim 12 (depends on 11)

The method of claim 11, wherein the phase object comprises two bars of PMMA (polymethyl methacrylate) material on a glass plate.

### Claim 13 (depends on 11)

The method of claim 11, wherein the phase object is a microfluidic channel and the Mach-Zehnder interferometer is utilized to observe liquid diffusion in the microfluidic channel.

### Claim 14 (depends on 8)

The method of claim 8, wherein the first and second gratings constitute a beam splitter/combiner, the method further comprising:
utilizing the beam splitter/combiner in a spectrometer-based optical coherence tomography (OCT) system;
the beam splitter/combiner splitting a collimated beam into a sample and reference beam;
a spherical lens L1 focusing an incident light beam and collecting back scattered light from a sample;
a spherical lens L2 collecting angularly dispersed reference light;
the back scattered light and the reference light undergoing a diffraction at the beam splitter/combiner where the back scattered light and the reference light are split and combined at output ports; and
lenses L3 and L4 collecting and focusing light from the output ports onto CCD (charge coupled device) arrays.