Dual balanced optical signal receiver

Claim 2

A dual balanced optical signal receiver, comprising: an optical mixing/polarization splitting means (101; 155-157; 173,175,176; 201; 211) for producing optical signals having an intermediate frequency (IF) by splitting a received optical signal and a local oscillation signal into two orthogonally polarized components and then mixing each of the thus split orthogonally polarized components to produce a plurality of mixed signals; a plurality of optical signal receiving means (102-105; 158-161; 177-180; 202,203; 212,213) for receiving said IF optical signals and transforming said IF optical signals into corresponding electric signals having an intermediate frequency (IF); said plurality of optical signal receiving elements (102-105; 158-161; 177-180; 202,203; 212,213) are connected with a plurality of subtracting means for receiving outputs from the plurality of optical signal receiving elements and producing a plurality of independent IF electric signals by subtracting, at least two IF electric signals corresponding to one polarization IF optical signal from each other and by subtracting at least two IF optical signals corresponding to the other polarization IF optical signal from each other; and a combining means (106; 162-164; 106; 206; 220) for receiving the independent IF electric signals and combining at least two of said independent IF electric signals having one type of polarization with at least two other independent IF electric signals having the other type of polarization.

Claim 3

A receiver as set forth in claim 1 or 2, wherein said receiver is further provided with; a modulation means (204; 214) for modulating said local oscillation optical signal to be input to said optical mixing/polarization splitting means, and a control means (205; 215) for extracting a modulation signal component from a signal output by said combining means and controlling an input or output of said optical signal receiving elements in such a manner that a level of the thus extracted modulation signal component is minimized.

Claim 4

A receiver as set forth in claim 1, wherein said optical mixing/polarization splitting means is comprised of a mixing part (113; 133) which mixes a received optical signal with a local oscillation optical signal to produce two mixed optical signals and a polarization splitting part (115,116; 135) which separates said two mixed optical signals into two orthogonally polarized components and produces a total of four mixed and split optical signals.

Claim 5

A receiver as set forth in claim 4, wherein a polarization direction of said local oscillation optical signal is adjusted in such a manner that the two orthogonally polarized components thereof have substantially a same intensity as each other.

Claim 6

A receiver as set forth in claim 5, wherein said local oscillation optical signal is supplied to said mixing part by way of a polarization-maintaining fiber (128,129) and a polarization plane thereof is rotated relative to a longitudinal axis thereof by an angle of 45°.

Claim 7

A receiver as set forth in claim 2, wherein said optical mixing/polarization splitting means is comprised of a polarization splitting part (155; 173) for receiving said received optical signal and said local oscillation optical signal and separating each of these two optical signals into two orthogonally polarized components and a mixing part (156,157; 175,176) which mixes the two polarized components having one polarization and mixes the other two polarized components having the other polarization, which polarized components are given from said polarization splitting part.

Claim 8

A receiver as set forth in claim 7, wherein a polarization direction of said local oscillation optical signal is adjusted in such a manner that the two orthogonally polarized components thereof have substantially the same intensity.

Claim 9

A receiver as set forth in claim 8, wherein said local oscillation optical signal is projected onto said polarization splitting part with a polarization plane rotated relative to the optical axis thereof by an angle of 45°.

Claim 10

A receiver as set forth in claim 4 or 7, wherein said plurality of optical signal receiving elements has four optical signal receiving elements which transform the four mixed and split optical signals from said mixing part into corresponding electric signals, respectively.

Claim 11

A receiver as set forth in claim 10, wherein said optical signal receiving elements are connected with said subtracting means for receiving outputs from said four optical signal receiving elements and producing two independent IF electric signals by subtracting, on one hand, two IF electric signals having one polarization from each other and on the other hand, subtracting the other two IF electric signals having the other polarization from each other.

Claim 12

A receiver as set forth in claim 11, wherein said combining means further contains a switching circuit (191) which selects from the two independent IF electric signals the independent IF electric signal having the better S/N ratio.

Claim 13

A receiver as set forth in claim 11 or 12, wherein said combining means (185) further contains a phase control means for matching the phases of two independent IF electric signals having the same polarization and the phases of the other independent two IF electric signals having the same polarization, and a summing means (186; 188) for summing the phase-matched two independent IF electric signals.

Claim 14

A receiver as set forth in claim 13, wherein said combining means further contains a signal intensity control means (184,185) for equalizing the intensity of the two independent IF electric signals and applying the same to said phase control means.