Four-port bidirectional optical circulator

A four-port optical circulator, comprising:

• at least first, second, third, and fourth optical input/output ports, first and second birefringent elements;
• a first polarization rotator optically coupled between the first and second birefringent elements, the first polarization rotator having one or more reciprocal polarization rotation sections and one or more optical path compensation sections;
• and a non-reciprocal polarization rotator;
• wherein the second birefringent element is disposed between the first polarization rotator and the non-reciprocal polarization rotator;
• wherein the second birefringent element is disposed between the first polarization rotator and the non-reciprocal polarization rotator;
• a retroreflector;
• wherein the non-reciprocal polarization rotator is disposed between the retroreflector and the second birefringent element;
• wherein the circulator routes optical signals from the first port to the second port;
• wherein the circulator routes optical signals from the second port to the third port;
• wherein the circulator routes optical signals from the third port to the fourth port;
• wherein the circulator routes optical signals from the fourth port to the first port.

A four-port optical circulator, comprising:

• at least first, second, third, and fourth input/output (I/O) ports;
• at least a first, second and third, birefringent element;
• at least a first polarization rotator optically coupled between the first and second birefringent elements, the first polarization rotator having one or more reciprocal polarization rotation sections and one or more optical path compensation sections;
• and
• at least a first, second, and third non-reciprocal polarization rotator,
• wherein the first non-reciprocal polarization rotator is disposed between the first polarization rotator and the second birefringent element;
• wherein the second birefringent element is disposed between the non-reciprocal polarization rotator and the second non-reciprocal polarization rotator;
• wherein the second non-reciprocal polarization rotator is disposed between the second and third birefringent elements;
• wherein the third birefringent element is disposed between the second and third non-reciprocal polarization rotators;
• wherein the circulator routes optical signals from the first I/O port to the second I/O port;
• wherein the circulator routes optical signals from the second I/O port to the third I/O port;
• wherein the circulator routes optical signals from the third I/O port to the fourth I/O port;
• and
• wherein the circulator routes optical signals from the fourth I/O port to the first I/O port.

A bi-directional optical add/drop apparatus, comprising:

• a first bi-directional circulator having first, second, third, and fourth input/out (I/O) ports;
• wherein the optical circulator routes optical signals from the second I/O port to the third I/O port;
• wherein the optical circulator routes optical signals from the third I/O port to the fourth I/O port;
• wherein the optical circulator routes optical signals from the fourth I/O port to the first I/O port.
• a first bandpass filter optically coupled to the second I/O port;
• and
• second bandpass filter optically coupled to the fourth I/O port
• wherein one or more of the first and second bandpass filters includes a filter selected from the group consisting of multi-dielectric layer coatings and cascaded band-stop fiber Bragg gratings.

A bi-directional optical add/drop apparatus, comprising:

• a first bi-directional circulator having first, second, third, and fourth input/output (I/O) ports;
• wherein the optical circulator routes optical signals from the second I/O port to the third I/O port;
• wherein the optical circulator routes optical signals from the third I/O port to the fourth I/O port;
• wherein the optical circulator routes optical signals from the fourth I/O port to the first I/O port,
• a first bandpass filter optically coupled to the second I/O port;
• a second bandpass filter optically coupled to the fourth I/O port;
• a second bi-directional circulator having first, second, third, and fourth input/output (I/O) ports;
• a third bandpass filter optically coupled to the second I/O port of the second bi-directional circulator;
• and
• a fourth bandpass filter optically coupled to the fourth I/O port of the second bi-directional circulator;
• wherein the third I/O port of the first bi-directional circulator is optically coupled to the first I/O port of the second bi-directional circulator.