Upgradeable Passive Optical Fiber Circuit

An optical circuit for combining and separating optical signals carried in optical fibers in each of a plurality of distinct optical channels defined by respective central wavelengths, the optical circuit comprising:

• first, second, and third optical interfaces;
• a first three-port combiner/separator filter coupled to the first optical interface and configured to pass optical signals in a first optical channel from the first optical interface to a first intermediate node while diverting optical signals in the remaining optical channels from the first optical interface to a second intermediate node, to pass optical signals in the first optical channel from the first intermediate node to the first optical interface while blocking optical signals in the remaining optical channels from the first intermediate node, and to pass optical signals in the first optical channel from the second intermediate node to the first intermediate node while diverting optical signals in the remaining optical channels from the second intermediate node to the first optical interface;
• a second three-port combiner/separator filter coupled between the first intermediate node, the second optical interface, and a third intermediate node, and configured to pass optical signals in the first optical channel from the second optical interface to the first intermediate node while diverting optical signals in the remaining optical channels from the second optical interface to the third intermediate node, to pass optical signals in the first optical channel from the first intermediate node to the second optical interface while blocking optical signals in the remaining optical channels from the first intermediate node, and to pass optical signals in the first optical channel from the third intermediate node to the first intermediate node while diverting optical signals in the remaining optical channels from the third intermediate node to the second optical interface;
• a third three-port combiner/separator filter coupled between the third intermediate node, the third optical interface, and a fourth intermediate node, and configured to pass optical signals in second and third optical channels from the third intermediate node to the third optical interface while diverting optical signals in the remaining optical channels from the second optical interface to the fourth intermediate node, to pass optical signals in the second and third optical channels from the third optical interface to the third intermediate node while blocking optical signals in the remaining optical channels from the third optical interface, and to divert optical signals in a fourth optical channel from the fourth intermediate node to the third intermediate node;
• and
• a three-port optical circulator coupled between the second and fourth intermediate nodes and an optical load, and configured to pass optical signals in at least the fourth optical channel from the second intermediate node to the fourth intermediate node and to pass optical signals in at least the remaining channels from the fourth intermediate node to the optical load.