Secure data transmission using spatial multiplexing

1. A method comprising:

receiving at a receiver a plurality of spatially multiplexed signals;

measuring a first differential value of a parameter between two of said signals and measuring a second differential value of said parameter between said two signals;

comparing the first and second differential values; and

setting an alarm indicator on the condition that an individual one of said first and second differential values, or a sum of said first and second differential values, changes by an amount greater than a pre-determined threshold.

2. The method of claim 1 wherein the measuring and comparing are performed optically.

3. The method of claim 1 wherein the measuring and comparing are performed electronically.

4. The method of claim 1 further comprising:

converting a plurality of modes received in the spatially multiplexed signals into a plurality of coded bit streams, wherein the number of coded bit streams is less than or equal to the number of modes; and

decoding information for one or more output bit streams from parity of the plurality of coded bit streams bits, wherein the plurality of coded bit streams for all the modes are provided at a same transmit wavelength and within a same symbol time slot.

5. The method of claim 4 wherein a subset of the plurality of modes which correspond to the one or more bit streams must be simultaneously and spatially selectively detected in order to recover a first transmitted bit stream.

6. An apparatus comprising:

a mode selective detector configured to detect spatially multiplexed signals;

a measurement module configured to measure a first differential value of a parameter between two of said signals and measuring a second differential value of said parameter between said two signals;

a difference calculator configured to compare the first and second differential values; and

a threshold and alarm module configured to activate an alarm indicator on the condition that an individual one of said first and second differential values, or a sum of said first and second differential values, changes by an amount greater than a pre-determined threshold.

7. The apparatus of claim 6 wherein at least one of the mode selective detector, the measurement module, the difference calculator and the threshold and alarm module are optical elements.

8. The apparatus of claim 6 wherein at least one of the mode selective detector, the measurement module, the difference calculator and the threshold and alarm module are electronic elements.

9. The apparatus of claim 6 further wherein at least one of the mode selective detector and the mode selective decoder is configured to decode information for one or more a plurality of coded bit streams derived from modes received in the spatially multiplexed signals from parity of the plurality of coded bit streams bits, wherein the plurality of coded bit streams for all the modes are provided at a same transmit wavelength and within a same symbol time slot.

10. The method of claim 1 , wherein the parameter is a power or a signal to noise ratio (SNR).

11. The method of claim 1 , wherein the plurality of spatially multiplexed signals is received via different modes of a multi-mode optical fiber.

12. The method of claim 1 , wherein the plurality of spatially multiplexed signals is received via different cores of a multi-core optical fiber.

13. The apparatus of claim 6 , wherein the parameter is a power or a signal to noise ratio (SNR).

14. The apparatus of claim 6 , wherein said detector is configured to receive the plurality of spatially multiplexed signals via different modes of a multi-mode optical fiber.

15. The apparatus of claim 6 , wherein said detector is configured to receive the plurality of spatially multiplexed signals via different cores of a multi-core optical fiber.