# US 2015/0146291 A1 · Faraday Rotator Mirror

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- Number: US 2015/0146291 A1
- Document ID: US20150146291A1
- Status: Published application
- Kind: A1
- Application number: 14/548,670
- Filed: 2014-11-20
- Published: 2015-05-28
- Earliest claimed: 2013-11-22
- Assignees: Oplink Communications, Inc.
- Applicants: Oplink Communications, Inc.
- Inventors: Hongwei Mao; Daxin Chen; Linhu Li; Jinyuan Qiao; Tanbin He; Zexiong Zhao; Feng Qing Zhou
- Web page: https://patentgrep.com/patent/US20150146291A1

## CPC

- G02F 1/0136: {for the control of polarisation, e.g. state of polarisation [SOP] control, polarisation scrambling, TE-TM mode conversion or separation (G02F1/0353 takes precedence)}
- G02B 6/272: {comprising polarisation means for beam splitting and combining}
- G02B 27/286: {for controlling or changing the state of polarisation, e.g. transforming one polarisation state into another (G02B5/3083 takes precedence; light guide coupling means utilising polarising elements G02B6/34)}
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

Methods, systems, and apparatus for optical communications. One of the apparatuses comprises a birefringent crystal configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams on respective paths to exit locations on the birefringent crystal; and a Faraday rotator positioned between the birefringent crystal and a beam folding optic assembly, wherein the Faraday rotator is positioned such that light beams exiting the birefringent crystal along a first path from a first exit location pass through the Faraday rotator before being incident on the beam folding optic assembly and that light beams exiting the birefringent crystal along a second path from a second exit location pass directly to the beam folding optic assembly without being incident on the Faraday rotator.

## Claims

### Claim 1

An apparatus comprising:
a birefringent crystal configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams on respective paths to exit locations on the birefringent crystal; and
a Faraday rotator positioned between the birefringent crystal and a beam folding optic assembly, wherein the Faraday rotator is positioned such that light beams exiting the birefringent crystal along a first path from a first exit location pass through the Faraday rotator before being incident on the beam folding optic assembly and that light beams exiting the birefringent crystal along a second path from a second exit location pass directly to the beam folding optic assembly without being incident on the Faraday rotator.

### Claim 2 (depends on 1)

The apparatus of claim 1, further comprising a collimator optically coupled between the birefringent crystal and an optical fiber, wherein light beams enter and exit the apparatus through the optical fiber.

### Claim 3 (depends on 1)

The apparatus of claim 1, wherein the beam folding optic assembly folds light beams incident along the first path to the reverse of the second path and wherein the beam folding optic assembly folds light beams incident along the second path to the reverse of the first path.

### Claim 4 (depends on 1)

The apparatus of claim 1, wherein the Faraday rotator rotates incident light beams by substantially 90 degrees.

### Claim 5 (depends on 1)

The apparatus of claim 1, wherein the birefringent crystal provides Faraday rotation angle filtering to provide suppression of error in the Faraday rotator.

### Claim 6

An apparatus comprising:
a polarization beam splitter configured to separate an incoming light beam input at a first port into component light beams having orthogonal polarization directions and directing the component light beams to respective second and third ports of the polarization beam splitter;
a Faraday rotator positioned between the polarization beam splitter and a first folding mirror such that light beams exiting the second port of the polarization beam splitter are directed through the Faraday rotator to the first folding mirror, and wherein the first folding mirror directs incident light beams from the Faraday rotator to a second folding mirror; and
the second folding mirror positioned between the polarization beam splitter and the first folding mirror such that light beams exiting the third port of the polarization beam splitter are reflected by the second folding mirror to the first folding mirror, and wherein first folding mirror directs incident light beams from the second folding mirror to the Faraday rotator.

### Claim 7 (depends on 6)

The apparatus of claim 6, further comprising a collimator optically coupled between the polarization beam splitter and an optical fiber, wherein light beams enter and exit the apparatus through the optical fiber.

### Claim 8 (depends on 1)

The apparatus of claim 1, wherein the first and second folding mirrors fold light beams incident along the first path from the second port of the polarization beam splitter to the reverse of the second path and wherein the first and second folding mirrors fold light beams incident along the second path from the third port of the polarization beam splitter to the reverse of the first path.

### Claim 9 (depends on 1)

The apparatus of claim 1, wherein the Faraday rotator rotates incident light beams by substantially 90 degrees.

### Claim 10 (depends on 1)

The apparatus of claim 1, wherein the of the polarization beam splitter provides Faraday rotation angle filtering to provide suppression of error in the Faraday rotator.

### Claim 11

A method comprising:
receiving a light beam, the components of the light beam having random polarization directions;
separating the light beam into a first beam and a second beam, the first beam following a first path and the second beam following a second path, wherein the first beam and the second beam have orthogonal polarization directions;
rotating the polarization direction of the first beam and then reflecting the first beam back along the second path;
reflecting the second beam back along the first path and then rotating the polarization direction of the second beam;
combining the first beam and the second beam such that the first beam and the second beam have orthogonal polarization directions, providing suppression of error in the rotation of the polarization direction of the first and second beams; and
outputting the combined beam.

### Claim 12 (depends on 11)

The method of claim 11, comprising collimating the received light beam before separating the light beam into a first beam and a second beam.

### Claim 13 (depends on 11)

The method of claim 11, wherein the light separating is performed by passing the light beam through a birefringent crystal.

### Claim 14 (depends on 11)

The method of claim 11, wherein the light separating is performed by passing the light beam through a polarization beam splitter.

### Claim 15 (depends on 11)

The method of claim 11, wherein the rotating the polarization direction is performed by passing the first beam through a Faraday rotator.